Frequently Asked Questions

Here is a wonderful page to help respond to climate change skeptics or deniers:


Climate change

Climate change affects each and every one of us. Its impacts are far-reaching, affecting potentially almost every aspect of our lives. Climate change has a direct effect on our health, our food and water sources, the air we breathe, and the weather we experience.

Climate change leads to extreme weather like hurricanes, floods, and wildfires and all the consequences these events carry with them. (link with detailed answer to this). Changing patterns of weather – more heat waves, droughts and altered precipitation patterns can lead to crop failures and food and water insecurity.

As climate continues to change, the risks to human health increase. For example, climate change promotes spreading vector-borne diseases as pests, such as mosquitos and ticks, expand their habitat and life cycles due to the rising temperatures. Allergy seasons are worsening, while the number of heart and respiratory health problems linked to the poor air quality and heat waves is increasing. This especially impacts certain parts of the population such as the elderly, children, and the poor.

Relatively small changes in the planet’s average temperature can lead to big changes in local and regional climate, creating risks to public health and safety, water resources, agriculture, infrastructure, and ecosystems. Climate change already has a serious impact on the world we live in today.

Every continent has warmed substantially since the 1950s. On average, there are more hot days, and the hot days are hotter. Heat waves have become longer and more frequent around the world over the past 50 years. This creates perfect conditions for extreme wildfire seasons around the globe.

Snow packs are melting earlier, leaving less water available during the heat of the summer. In some areas, this leads to reduced amounts of available freshwater, affecting major cities with droughts. Melting of sea ice and glaciers is also raising the global sea level.

Precipitation patterns are also changing. Air can hold more moisture as it warms. As a result, storms and floods are getting stronger and more frequent. This has a major impact on crops, some foods are becoming less nutritious. Increased atmospheric CO2 speeds up photosynthesis, the process that helps plants transform sunlight to food. While this makes plants grow faster, in doing so, they pack in more carbohydrates like glucose at the expense of other essential nutrients.

Many land and marine species have had to shift their geographic ranges in response to warmer temperatures. While some species may adapt to rapidly changing the land, freshwater, and marine habitats, others will suffer population declines, collapse and even extinctions.

Humans will suffer when some ecosystems no longer provide the services (food, coastal defense, clean water, etc.) we depend on. A major concern with the current episode of warming is that it is happening so rapidly that humans and nature might have insufficient time to adapt. Entire ecosystems, communities, and even countries are at great risk. Much of the human population lives in coastal areas that will be inundated by higher seas and larger storms, with property losses that will total billions of dollars in this century. Climate change is already prompting an increase in migration, with people being forced to leave their homes because of drought, flooding, and other climate-related disasters.

The Earth’s future climate will depend on whether we manage to slow or even reduce greenhouse gas emissions, but warming is likely to continue.

A rise in global temperatures increases the severity and likelihood of storms, floods, wildfires, droughts and heat waves. Climate change affects the weather by intensifying the water cycle. Water evaporates into the atmosphere from both land and sea and returns to Earth’s surface in the form of rain and snow. As the temperatures are rising, the rate of evaporation from our oceans is increasing. This creates perfect conditions for strong storms and hurricanes. Over the past 20 years, tropical storm activity in the Atlantic Ocean, Caribbean, and the Gulf of Mexico has increased in intensity, frequency, and duration. Increased rates of evaporation on land can lead to more rapid drying of soils and severe droughts. The extent of regions affected by droughts has also increased as precipitation over land has marginally decreased while evaporation has increased due to warmer conditions.

Global warming contributes to rising sea levels in two ways. First, hotter summers, warmer winters, and earlier springs are causing glaciers and ice sheets to gradually melt. The increased runoff from polar lands is causing sea levels to rise. Second, thermal expansion, the natural expansion of water as it heats up, is causing the ocean to take up more space, which also leads to rising sea levels.

Scientific calculations show decades of more ice losses than gains. On average, most of Earth’s mountain glaciers are continuing to melt. The Earth’s polar regions are especially vulnerable to global warming because temperatures in the Arctic and Antarctic are rising at twice the rate of the world average. Arctic and Antarctic sea ice volume and extent have been declining since record-keeping began in the late 1970s and prior.

Due to time lags in the climate system and the fact that CO2 stays in the atmosphere for hundreds or thousands of years, the climate will continue to warm until at least mid-century regardless of what we do today to reduce emissions. If we fail to make substantial cuts to greenhouse gas emissions, the Earth will keep warming for centuries to come.

The solutions to the climate crisis are numerous, but the most important goal is the urgent action to cut greenhouse gas emissions. This will require stepping up improvements in energy efficiency, reducing waste, slowing deforestation, and shifting to cleaner energy sources.

It requires global efforts such as international policies and agreements between countries, local efforts on the city- and regional level, but it is also a matter for personal action. There are many actions that individuals and business can take to reduce their carbon footprint and act on climate change. Simple actions such as using energy-efficient light bulbs and appliances, recycling and composting, purchasing green power, using public transit, and bicycling or walking instead of driving can make a difference by reducing your household’s carbon footprint.

Learn more

1. IPCC (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change.

2. IPCC (2007). Climate Change 2007: The Physical Science Basis. Frequently Asked Questions. FAQ 7.1. Intergovernmental Panel on Climate Change.

3. IPCC (2007). Climate Change 2007: The Physical Science Basis. Executive Summary. Intergovernmental Panel on Climate Change.

4.​ USGCRP (2016). The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. Crimmins, A., J. Balbus, J.L. Gamble, C.B. Beard, J.E. Bell, D. Dodgen, R.J. Eisen, N. Fann, M.D. Hawkins, S.C. Herring, L. Jantarasami, D.M. Mills, S. Saha, M.C. Sarofim, J. Trtanj, and L. Ziska, Eds. U.S. Global Change Research Program.

5. USGCRP (2014).Climate Change Impacts in the United States: The Third National Climate Assessment. Melillo, Jerry M., Theres (T.C.) Richmond, and Gary W. Yohe, Eds., U.S. Global Change Research Program.

6. National Research Council (2011). Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millenia. National Academies Press.

Climate is not changing for the first time. There have been fluctuations and changes in climate again and again over geological time. This might lead to the impression that current climate change is a recurrent, harmless event in (Earth) history.

On the other hand, paleontological evidence suggests that major changes in climate are associated with ecological catastrophes, including mass extinctions. Every species has its own climatic window it can tolerate. If it is getting warmer or colder, many individuals of a species might die and species have to either successfully relocate or go extinct. Extinctions might also happen due the unforeseeable new interactions between those species that can find habitat in the new world. In the oceans, other results of climate change, such as acidification and anoxia might have devastating consequences for the biota.

Due to human activities, climate warms at a previously unprecedented rate. This leaves even shorter time for ecosystems to adapt to the physical changes of the environment than what we observed in the fossil record. Based on this, there is a considerable chance that the current climate change will provoke a mass extinction which can destroy our existence and livelihood that depends on the healthy ecosystems.

The difference between weather and climate lies in the time frame in which the data are collected and analyzed. Weather represents all current processes in the atmosphere, including temperature and precipitation data measured over days. Climate, on the other hand is related to weather data over at least a 30 year time period and also accounts for normal frequency of extreme weather events. Extreme weather events are not caused by the climate, but if the weather becomes more extreme over a long-time space, this will be an indication for a drastic change in climate.


Because of global warming, warm air masses are moving further north and significantly warm the Arctic. Simultaneously, cold air masses are repressed which move southward. In addition, the increasing melt water can repress warm sea currents into the South. It becomes less cold in the polar regions and warmer in the southern regions. This is comparable with an open refrigerator from which the cold air escapes. If this- cold air disappeared, the climate will also warm in those affected regions. These cold waves form the disappearing cold from the Arctic.

Climate models

Global climate models or General Circulation models (GCMs) are the most complex and precise models for understanding climate systems and predicting climate change. These models aim to mathematically describe the Earth’s climate system based on the laws of physics (e.g. first law of thermodynamics, Stefan-Boltzmann law), fluid motion (e.g. Navier-Stokes equations) and chemistry. They use mathematical equations to quantify observable Earth system processes, i.e. characterize how energy and matter interact and get transported in different parts of the atmosphere, land, ocean and sea ice (Fig. 1).

The atmospheric component of the climate model simulates clouds, aerosols and the transport of heat and water around the globe. The land surface component simulates surface characteristics such as vegetation, snow cover, soil water, rivers, and carbon storing, whereas the ocean component simulates current movement, mixing and ocean biogeochemistry. The sea ice component modulates solar radiation absorption, air-sea heat and water exchanges.

Building a complex global climate model incorporating all these components requires the division of the Earth’s surface into three dimensional grid cells (Fig. 1). The size of these grid cells defines the spatial resolution of the model (typically about 100 km x 100 km x 30 vertical layers). Climate models also incorporate the dimension of time, measured in time steps. The temporal resolution refers to the size of these time steps (typically about 30 minutes) used in the model. Powerful supercomputers iteratively solve the mathematical equations for every single spatial grid cell and step the model forward in time to produce a precise climate model for a specific time interval. Models with smaller grid cells as well as smaller time steps lead to better resolution, but also need considerably more computing power.

Fig. 1: Schematic representation of GCMs (https://www.gfdl.noaa.gov/climate-modeling/)



The main inputs for a climate model are external factors, so called “forcings”, that change the amount of the sun’s energy absorbed by the Earth or trapped in the atmosphere. Examples of these forcings are the sun’s varying radiation output, variable atmospheric concentrations of greenhouse gasses (e.g. CO2, methane, N2O) or aerosols (particles emitted e.g. by fossil fuel burning and volcanic eruptions influencing sunlight and cloud formation). These factors are incorporated into the climate model as best estimates of past conditions or as part of future socio-economic and emission scenarios.

Past forcings can be estimated by reconstructing ancient greenhouse gas concentrations (e.g. by analyzing air trapped in ice cores), climate gas and particle emissions during past volcanic eruptions or changes in the Earth’s orbit (i.e. cyclical variations in solar radiation reaching the Earth due to Milankovitch cycles).

Concerning future forcings, different scenarios of future developments in technology, energy and land use provide potential  pathways, so called “Representative Concentration Pathways” (RCPs), for atmospheric greenhouse gas concentrations (Fig. 1).

Fig. 1: Future trends in concentrations of greenhouse gases based on different RCP scenarios assuming different amounts of radiative forcing (van Vuuren et al., 2011).

The main outputs for a climate model are normally temperatures and humidity of different atmospheric layers from the surface to the upper stratosphere. Climate models also produce estimates of ocean temperatures, salinity and pH from the surface to the seafloor as well as snowfall, rainfall, snow cover and the extent of glaciers, ice sheets and sea ice. They also give information about wind speed, strength and direction, as well as climate features, such as the jet stream and ocean currents. “Climate sensitivity” can also be modelled (i.e. the warming expected when the concentration of carbon dioxide in the atmosphere reaches twice the amount it was in preindustrial times).


van Vuuren, D. P., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., Hurtt, G. C., Kram, T., Krey, V., Lamarque, J.-F., Masui, T., Meinshausen, M., Nakicenovic, N., Smith, S. J., and Rose, S. K., 2011, The representative concentration pathways: an overview: Climatic Change, v. 109, no. 1, p. 5.

Climate models are tested by comparison of model predictions with real-world observations. For this purpose, climate models are run over a historical period, from around 1850 to near-present, using best estimates for the past forcings  during this time period (also see “What are the inputs and outputs for a climate model?”). These “hindcasts” of the past climate (e.g. surface temperatures)are then compared to actual recorded climate observations (Fig. 1c). The more precise the hindcast of past climate, the more reliable is the climate model, also in forecasting future climate.

These historical “hindcast” runs can also be used to determine human influence on climate change, the so called “attribution”. For this purpose, models are run with either only natural forcings (e.g. solar variation and volcanic activity) or anthropogenic forcings (e.g. greenhouse gasses and aerosols) as model inputs (Fig. 1a, b). These graphs show that natural forcings alone can’t explain climate’s behavior. Only when we also take anthropogenic forcing into account, we can explain the observed climate patterns.

Fig. 1: Comparison of model results with recorded climate observations: (a) climate model with only natural forcings as input, (b) climate model with only anthropogenic forcing as input, (c) climate model with both natural and anthropogenic forcings as input (https://www.ipcc.ch/report/ar3/wg1/summary-for-policymakers/spmfig04/).

Furthermore, big perturbation events like volcanic eruptions can be used to test climate model performance. Model projections can be compared to recorded short-term climate responses after an eruption. Studies focusing on the Mount Pinatubo eruption show that models can accurately project changes in temperature (Hansen et al., 1996) and atmospheric water vapor (Soden et al., 2002).

To produce more reliable estimates of twenty-first century climate, climate models are also tested against paleoclimate data (reaching back up to 21,000 years). This data shows larger climate changes than the observational record of the last 150 years, against which climate models are normally evaluated. Ice-core, marine (e.g. marine sediments) and terrestrial archives (e.g. tree rings) provide information about environmental responses to past climate changes. These records can be used to derive estimates of climate, i.e. provide paleo-proxies for past climate (e.g. paleotemperatures). Thus, the geologic record provides a unique opportunity to test model performance outside of the comparison with the short-term observational record. Evaluation of model simulations against paleodata shows that models reproduce the direction and large-scale patterns of past changes in climate, even though they tend to underestimate the magnitude of regional changes (Braconnot et al., 2012).


Braconnot, P., Harrison, S. P., Kageyama, M., Bartlein, P. J., Masson-Delmotte, V., Abe-Ouchi, A., . . . Zhao, Y. (2012). Evaluation of climate models using palaeoclimatic data. Nature Climate Change, 2(6), 417-424.
Hansen, J., Sato, M., Ruedy, R., Lacis, A., Asamoah, K., Borenstein, S., . . . Campbell, M. (1996). A Pinatubo climate modeling investigation. In The Mount Pinatubo Eruption (pp. 233-272): Springer.
Soden, B. J., Wetherald, R. T., Stenchikov, G. L., and Robock, A., 2002, Global Cooling After the Eruption of Mount Pinatubo: A Test of Climate Feedback by Water Vapor: Science, v. 296, no. 5568, p. 727-730.

Modern climate models can generally be considered reliable tools for predicting climate. A recent study (Hausfather et al., 2020) evaluated the performance of various climate models published between the early 1970s and the late 2000s. They looked at how well models project global warming in the years after they were published by comparing the model projections to actual observed temperature changes. 14 out of the 17 model projections were consistent with observation, especially when mismatches between projected and observationally-informed estimates of forcing were taken into account. This means that the actual climate physic models were generally accurate and mismatches between output model temperatures and observed climate data occurred mainly due to uncertainties in future forcing estimates, i.e. estimates of future climate gas emissions, which need to be put into the climate model (also see “What are the inputs and outputs for a climate model?”).


Hausfather, Z., Drake, H. F., Abbott, T., and Schmidt, G. A., 2020, Evaluating the Performance of Past Climate Model Projections: Geophysical Research Letters, v. 47, no. 1, p. e2019GL085378.

As computational power is limited, there is a lower limit to the grid cell size for which climate models can be calculated (see also “What is a climate model?”). However, there are processes at scales below the model’s spatial resolution (normally around 100 x 100 km), e.g. clouds, convection in the atmosphere, eddies in the ocean, land surface processes (Fig. 1). The physics of these processes needs to be “parameterized”. These parameterizations are approximations of the specific phenomena to be modelled, at the scales the model can actually resolve. Parameterization is also used as an approximation of climate processes which are not yet fully understood. Parameterizations are the main source of uncertainty in climate models.

Fig. 1: Climate processes and properties that typically need to be parameterized within global climate models (MetEd, The COMET Program, UCAR).

As our knowledge of the climate as well as our empirical observations are incomplete, we cannot always narrow down parameterized variables into a single value. Therefore, tests with the model are run. Estimations of parameterized variables are put into the model to find the value, or set of values, that give the best representation of the climate. This process is called “model tuning”. Modelers tune their models to ensure that the long-term average state of the climate is accurate – including factors such as absolute temperatures, sea ice concentrations, surface albedo and sea ice extent.

There are also some limitations associated with modelling climate at regional and local scales. To bridge the gap from the large spatial scales represented by GCMs to the smaller scales required for assessing regional climate change and its impacts, different downscaling methods are used. There are two ways of downscaling: regional climate models (RCMs) and empirical-statistical downscaling (ESD). Regional climate models (RCMs) take the low-resolution solution provided by the GCMs and include finer topographical details such as the influence of lakes, mountain ranges and a sea breeze to calculate more detailed information. These models can achieve a resolution of around 25km x 25km. As it is the larger scale model information that drives the finer-scale model, this approach only provides limited improvability of the data.

Empirical-statistical downscaling (ESD) is an alternative that does not require much computing power. ESD uses observed climate data to establish a statistical relationship between the global and local climate. According to this relationship, local changes can be derived based on the large scale projections coming from GCMs or observations.

Both RCMS and ESD give relatively consistent results with each other as well as with observed data (Fig. 2). However, RCMs as well as ESD downscaled information relies heavily on the quality of the information that it is based on, i.e. the observed data or the GCM data input. Downscaling only provides more location-specific data, it does not make up for any uncertainties that stem from the data or GCM it relies on.

Fig. 2: A comparison between RCM results based on different climate models (colored dots with error bars) and ESD results (red region showing the 90% confidence interval for the model ensemble), actual observations are shown as black symbols (Førland et al., 2011).

Global as well as downscaled climate models can simulate climate quite accurately, but sometimes they show substantial deviations from observed climate, known as “bias”, especially at the regional and local scale. Bias is defined as the systematic difference between a modelled climate property (e.g. mean temperature) and the corresponding real property. Bias correction can be applied to account for these differences. An empirical transfer function between simulated and observed climate properties is calibrated and applied to the model output data to match observational climate data. Bias correction is a mere post-processing and cannot fix problems with the actual climate model.

Individual climate models may also struggle to accurately depict natural climate variability, i.e. natural short-term fluctuations on seasonal or multi-seasonal time scales (e.g. North Atlantic Oscillation (NAO) or El Niño Southern Oscillation (ENSO)). However, when combining several independent models, this variability can be reduced. Averaging an ensemble of different climate models can produce forecasts, which show better skill, higher reliability and consistency in predicting climate (Hagedorn et al., 2005).

In conclusion, modern climate models can definitely provide reliable projections at larger, global scales. However, they reach their limits when having to deal with small scale processes at regional or local scale and short-term climate variability. To deal with these problems, there are some effective methods available (as described above). Even though models will never predict our climate system 100% accurately, they are definitely still skilled in giving us a reasonably precise prediction of future climate; or, to put it in George Box’s words: “All models are wrong, but some are useful”.


Førland, E. J., Benestad, R., Hanssen-Bauer, I., Haugen, J. E., and Skaugen, T. E., 2011, Temperature and precipitation development at Svalbard 1900–2100: Advances in Meteorology, v. 2011.
Hagedorn, R., Doblas-Reyes, F. J., and Palmer, T. N., 2005, The rationale behind the success of multi-model ensembles in seasonal forecasting – I. Basic concept: Tellus A, v. 57, no. 3, p. 219-233.

Understanding past, present and future climate and combining this knowledge with climate models helps to determine natural as well as man-made influences on the climate system of the past and the future (also see“How are climate models validated? How are they tested?”). Climate projections are helpful in assessing the impact of future climate change and assist decision-makers to prioritize environmental issues based on scientific evidence (Fig. 1).

Fig. 1: Climate change vulnerability (Wesleyan University and Columbia University).

Therefore, it is important to continue to collect data and improve recent models to increase their accuracy and refine our knowledge of the climate system. Climate models have the ability to influence the way communities and policy-makers plan for the future. These models are our best chance at finding ways to mitigate the dangerous effects of climate change.



Global Warming

The terms “global warming” and “climate change” are sometimes used interchangeably, but strictly speaking they refer to slightly different things. Global warming refers to the warming of the Earth over longer time periods. Due to documentations of global temperature, an increase is observed since the early 20th century and especially since the late 1970s. A worldwide average temperature increase of around 1°C has been documented since 1880, relative to the mid-20th-century baseline (of 1951 – 1980). This is on top of an additional 0.15 °C of warming from between 1750 and 1880.

Climate change on the other hand includes global warming but refers to the larger range of environmental changes. These changes include rising sea levels, retreating mountain glaciers and accelerating ice melt in Greenland, Antarctica and the Arctic.



Current climate change is mainly caused by the “greenhouse effect”, where heat-trapping gases (water, carbon monoxide, carbon dioxide, methane, ozone and nitrous oxide) in the atmosphere cause an increase in global temperature. The effect occurs due to the atmosphere being almost completely permeable for solar short-wave radiation, but less transparent for the long-wave infrared radiation. The short-wave radiation is absorbed by the surface and emitted as infrared radiation, which then can be absorbed by the greenhouse gases. The gases then release the radiation once more towards space and towards the Earth’s surface. This effect traps part of the solar energy in between the atmosphere and the surface and contributes to an increase in global temperature.

Effectiveness of greenhouse gases

The intensity of the greenhouse effect depends on the atmosphere’s temperature and on the amount of greenhouse gases that the atmosphere contains. Without incorporating the effects of clouds, the largest portion of the greenhouse gases is made up of water vapor in the atmosphere with a percentage of 36 to 70%. Carbon dioxide contributes with 9 to 26%, methane with 4 to 9 % and tropospheric ozone with around 3 to 7% to the global greenhouse effect. It is not possible to assign exact percentages for the different greenhouse gases, because the absorption and emission bands of the gases overlap. Clouds also absorb and emit infrared radiation and thus affect the radiative properties of the atmosphere.

Greenhouse gases throughout history

Greenhouse gases have been around since the formation of Earth’s atmosphere. These naturally arising greenhouse gases add to a comfortable global mean temperature of +15°C and contributed to evolution of life as we know it. Without an atmosphere, Earth would be a much colder place, with a mean temperature of around -18°C. Since the industrial revolution, however, the anthropogenic input of greenhouse gases into the atmosphere has increased significantly. Anthropogenic influences, like the burning of fossil fuels, deforestation, production of cements and agriculture emit long-lived greenhouse gases (carbon dioxide, methane and nitrous oxide) as well as tropospheric ozone, which amplify the greenhouse effect. Measurements of CO2 from the Mauna Loa observatory show that the concentrations have increased from about 313 parts per million (ppm) in 1960 to 400 ppm in May 9, 2013 (figure 1). The global concentration of greenhouse gases has increased constantly since.

Figure 1: The Keeling Curve of atmospheric CO2 concentrations measured at Mauna Loa Observatory. Figure adopted from wikipedia.org.




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The greenhouse effect can be explained in several steps. It all begins with the sun emitting electromagnetic waves in form of ultraviolet, visible and near-infrared radiation towards the earth, with a common wavelength of around 500 nm. This wavelength maximum of the spectrum is only scarcely absorbed by the atmosphere, since greenhouse gases like water, carbon dioxide, methane and ozone are permeable for the short-wave radiation. The atmosphere and clouds reflect around 26% of the solar energy back into space and absorb 19%. After passing the atmosphere, the remaining solar energy hits the Earth’s surface, where a small part of the energy is reflected back into space, while the rest is absorbed. The photons induce their energy into the surface and cause a heating effect. The heated surface then emits infrared radiation with a wavelength of 10.000 nm.

Figure 2: Diagram showing light energy (white arrows) emitted by the sun, warming the earth’s surface which then emits the energy heat (orange arrows), which warms the atmosphere and is then emitted as heat by three of the greenhouse gas molecules (water, carbon dioxide and methane). Figure adopted from wikipedia.org.

The long-wave infrared radiation emitted by the surface is more likely to be absorbed by greenhouse gases in the atmosphere. Thus, only a small part of this radiation escapes the atmosphere back into space. The infrared radiation can be absorbed and emitted by greenhouse gases due to their molecular structure with two different atoms (carbon monoxide) and all gases with three or more atoms. The energy absorbed by the greenhouse gases causes the loosely bound molecules to vibrate and, at some point, release the radiation again. This energy is then emitted evenly into space or back to the surface. Hitting the surface, this energy is absorbed again, and an additional heating effect occurs due to the energy being trapped in the lower atmosphere (figure 2). Increasing the concentration of greenhouse gases increases the amount of absorption and reradiation and thereby further warms the atmosphere and the surface below. Around 99% of the dry atmosphere is infrared transparent because the main constituents are nitrogen, oxygen and argon. These gases are composed of either one atom or two identical atoms, thus these gases are not able to directly absorb or emit infrared radiation. Intermolecular collisions, however, cause the energy absorbed and emitted by the greenhouse gases to be shared with the non-infrared active gases. Even though the atmosphere is composed largely of non-reactive gas molecules, the small amount of greenhouse gases has a huge impact on global warming and by increasing their concentration, the greenhouse effects develops a positive feedback loop, which results in temperatures increasing more quickly.

Effects of the albedo

As already mentioned above, one important factor that contributes to the greenhouse effect is the ability of the Earth’s surface to absorb or reflect the solar radiation. These parameters are described by the albedo, which determines the measure of the diffuse reflection of solar radiation out of the total solar radiation received by the Earth. This effect can be explained with a simple example. Snow reflects a lot of sunlight, resulting in lower heat gain for the surface and thus, less warming. If the area of the snow cover decreases, the proportion of reflected sunlight decreases, resulting in higher heat gain and therefore warming.

The albedo is a dimensionless parameter, measured on a scale from 0, which describes a black body that absorbs all radiation, to 1, corresponding to a body that reflects all incident radiation. The average albedo of Earth from the upper atmosphere is around 0.3 – 0.35 because of the cloud cover, but widely varies locally across the surface due to different geological and environmental features. The average albedo of 0.3 – 0.35 means that 30 – 35% of the incoming solar radiation is reflected by the surface. The most important surface albedos are described by the oceans (0.06), forests (0.08 to 0.18), the continental surface (0.1 to 0.4), ocean ice (0.5 to 0.7) and fresh snow (0.8). The albedo varies with latitude, being highest near the poles and lowest in the subtropics, with a local maximum in the tropics. Albedo affects climate by determining how much radiation is absorbed by the planet. Uneven heating from albedo variations between land, ice or ocean surfaces can drive weather.

Climate change and albedo

The intensity of albedo temperature effects depend on the amount of albedo and the level of local insolation (solar irradiance). The Arctic and Antarctic regions are cold due to low solar irradiance and high albedo, whereas also higher albedo areas like the Sahara Desert are hotter due to higher insolation. Arctic regions notably release more heat back into space than what they absorb, effectively cooling the planet, but snow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to 0.4 for melting snow and 0.2 for dirty snow. With the current climate change, arctic ice and snow are melting at higher rates due to increasing temperatures. With the warming of snow-covered areas, the snow tends to melt, lowering the albedo and hence leading to more snowmelt because more radiation is being absorbed. As a consequence of the melting the underlying surface is exposed (water or ground with lower albedo), which then results in even less solar radiation being reflected back into space. This process creates a positive feedback loop, which results in a reduced albedo effect. In summary, the lower the reflection of the solar radiation, the more infrared radiation is produced. The higher the amount of infrared radiation, the stronger is the effect of greenhouse gases resulting in global temperatures to increase.




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“The thawing Arctic threatens an environmental catastrophe”. The Economist. 29 April 2017. Retrieved 8 May 2017.

Betts, RA (2000). “Offset of the potential carbon sink from boreal forestation by decreases in surface albedo”. Nature. 408 (6809): 187–190. Bibcode:2000Natur.408..187B. doi:10.1038/35041545. PMID 11089969.

Boucher; et al. (2004). “Direct human influence of irrigation on atmospheric water vapour and climate”. Climate Dynamics. 22 (6–7): 597–603. Bibcode:2004ClDy…22..597B. doi:10.1007/s00382-004-0402-4.

Climate sensitivity is a measure used for climate modeling, which describes the amount of warming in the atmosphere associated with increases in atmospheric carbon dioxide (CO2) as a result of the anthropogenic greenhouse effect. Even though only a tiny amount of the gases in Earth’s atmosphere are greenhouse gases, they have a huge effect on climate. Climate sensitivity indicates how much the Earth’s surface temperature would increase if pre-industrial CO2 concentrations were doubled.

Svante Augustus Arrhenius was the first to calculate global climate sensitivity to changes in atmospheric CO2 concentration. Arrhenius calculations showed temperature increases with 4°C, which are quite close to modern estimates. According to modern reports by the Intergovernmental Panel on Climate Change (IPCC), the possible range of climate sensitivity to doubling of atmospheric CO2 is from 1.5 to 4.5°C and the probable date of CO2 doubling will be reached between the years 2050 and 2100.

Estimating climate sensitivity

Climate sensitivity is typically estimated in three ways; by using observations taken during the industrial age, by using temperature and other data from the Earth’s past, and by modeling the climate system. There are two types of climate sensitivity: Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR).

The ECS is the amount of warming achieved when the entire climate system reaches equilibrium to a doubling of CO2. The ECS is likely to be 1.5°C to 4.5°C and extremely unlikely to be less than 1°C and not greater than 6°C. These estimations for temperatures are repeated in assessment reports every 6 years. These reports have so far shown a good consistency in the temperature values with only minor variations.

The TCR considers the changes that would occur if CO2 levels increase by 1% per year until they double. If atmospheric CO2 concentrations were held at double pre-industrial concentrations, the planet would still continue to warm. This is because the world’s oceans take a long time to heat up in response to the enhanced greenhouse effect. The TCR is likely to be 1°C to 2.5°C and extremely unlikely to be greater than 3°C.


IPCC Report 2013 : https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=2ahUKEwiM9JGg6rXmAhVpyqYKHTSqD0gQFjAFegQIDBAI&url=https%3A%2F%2Fwww.environment.gov.au%2Fsystem%2Ffiles%2Fresources%2Fd3a8654f-e1f1-4d3f-85a1-4c2d5f354047%2Ffiles%2Ffactsheetclimatesensitivitycsiro-bureau.pdf&usg=AOvVaw3WoW0uio9aS1VHbn34OiOR



Held, Isaac and Winton, Mike. Transient and Equilibrium Climate Sensitivity, Geophysical Fluid Dynamics Laboratory, U.S. National Oceanic & Atmospheric Administration. Retrieved 2019-06-19.

PALAEOSENS Project Members (2012). “Making sense of palaeoclimate sensitivity” (PDF). Nature. 491 (7426): 683–91. Bibcode:2012Natur.491..683P. doi:10.1038/nature11574. hdl:2078.1/118863. PMID 23192145.

Rahmstorf, Stefan (2008). “Anthropogenic Climate Change: Revisiting the Facts”. In Zedillo, Ernesto (ed.). Global Warming: Looking Beyond Kyoto (PDF). Brookings Institution Press. pp. 34–53.

Previdi, M.; et al. (2013). “Climate sensitivity in the Anthropocene”. Quarterly Journal of the Royal Meteorological Society. 139 (674): 1121

Through Earth’s history, the climate has changed considerably over a wide range, from ice ages to warm periods without any ice at the poles. These climate variations had several triggering mechanisms, like variation in solar activity, Milanković cycles, volcanic activity and changes in atmospheric composition. With data from ice cores, scientists were able to recover atmospheric data from the past 400,000 years. The data from these ice cores included the annual atmospheric composition over the period of these 400,000 years, with the different concentrations of atmospheric gases. After analyzing the data, a correlation between CO2 and temperature can be reconstructed (see figure below). However, a rise in CO2 did not precede a rise in temperature but is lagging behind the temperature curve by 200 to 1000 years. At the first glance, the temperature seems to be the driving factor causing the increase in CO2 concentration, which is contrary to the common point of view that CO2 drives global warming. A study by Shakun et al. (2012) investigates this correlation in detail.

Figure 3: Vostok ice core records for carbon dioxide concentration and temperature change. Figure adopted from skepticalscience.com.

The study shows that the initial stages of increasing temperatures after the last ice age were triggered by the Milanković cycles. This initiated a reaction chain, leading to the heating of the oceans, which then released CO2. With the increasing greenhouse effect, temperatures started to increase and the emission of CO2 from the oceans into the atmosphere accumulated. The time lag between CO2 and temperature is caused by a temporal offset between the oceans heating up and the constant release of oceanic CO2. Through this accumulating effect, CO2 became the primary driver of temperature during the glacial-interglacial warming. The increasing CO2 levels then become both, the cause and effect of further warming. This positive feedback is necessary to trigger the shifts between glacials and interglacials as the effect of orbital changes is too weak to cause such variation.


The main statement of Shakun et al. (2012) is that CO2 has led global warming, rather than following it. The actual interactions between temperature and CO2 described in this study are more complex. The data acquired for this study is based on an Antarctic ice core record for atmospheric CO2 data as well as sediment cores from around the globe, which extend back to the last glacial-interglacial transition 18,000 years ago. From these proxy records, sea surface temperatures for marine records and surface air temperatures were collected.

Comparing the CO2 increase to the different temperature records it is possible to estimate whether CO2 led or lagged temperature in different geographic distributions. The result was that CO2 lags and leads temperature changes at the same time. The southern hemisphere showed temperature rising before CO2 concentration, while the northern hemisphere showed the opposite (see figure below). This result can be explained by several factors.

The initial warming, which was triggered by the Milanković cycles, is reflected in the highest latitudes and started approximately 19,000 years ago. The arctic warming then melted large quantities of ice, causing a strong freshwater input into the oceans. This influx of fresh water disrupted the Atlantic meridional overturning circulation (AMOC), which in return caused a seesawing of heat between the hemispheres. The AMOC is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic and a southward flow of colder, deep waters that are part of the thermohaline circulation. The bipolar seesaw concept describes the anti-phasing of Greenland and Antarctic temperature changes along with Dansgaard-Oeschger climatic oscillations and Heinrich Events during the last glacial period. Sudden changes in the thermohaline circulation affect the polar climate in both hemispheres through changes in the south-northward heat transport. With a freshwater influx in the North Atlantic, the thermohaline circulation (AMOC) is turned off and the northern hemisphere cools down, while the southern hemisphere and tropics heat up at around 18,000 years ago. Once the deep-water formation turns on again, the meridional heat transfer resumes and the northern hemisphere warms while the southern hemisphere loses heat.

Figure 4: CO2 concentration ratio and temperature. (Figures adopted from Shakun et al. 2012). a) The global proxy temperature stack (blue) as deviations from the early Holocene (11,5 – 6,5 kyr ago) mean, an Atlantic ice-core composite temperature record (red), and atmospheric CO2 concentrations (yellow dots). The Holocene, Younger Dryas (YD), Bølling-Allerød (B-A), Oldest Dryas (OD) and Last Glacial Maximum (LGM) intervals are indicated. Error bars, 1-sigma; p.p.m.v. = parts per million by volume. b) The phasing of CO2 concentration and temperature for the global (grey), northern hemisphere (NH; blue) and southern hemisphere (SH; red) proxy stacks based on lag correlations from 20 – 10 kyr ago in 1,000 Monte Carlo simulations. The mean and 1-sigma of the histograms are given. CO2 concentration leads the global temperature stack in 90% of the simulation and lags it in 6%.

The warming of the southern ocean 18,000 years ago caused a decrease of CO2 solubility in the water. The lower solubility resulted in the emission of CO2 from the ocean into the atmosphere at around 17,500 years ago, which in return caused a global warming due to the greenhouse effect. This time lag of around 500 years is what causes the CO2 lagging behind temperature in the ice core record. With the increasing CO2 concentrations, deriving from the heating oceans, the greenhouse effect starts accumulating, making CO2 the driver of further temperature changes.





Knutti, R., Flückiger, J., Stocker, T.F. and Timmermann, A., 2004. Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation. Nature, 430, 851-856. 

 Shakun, Jeremy D.; Clark, Peter U.; He, Feng; Marcott, Shaun A.; Mix, Alan C.; Liu, Zhengyu et al. (2012): Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. In Nature 484 (7392), pp. 49–54. DOI: 10.1038/nature10915.

Muryshev, K. E.; Eliseev, A. V.; Denisov, S. N.; Mokhov, I. I.; Arzhanov, M. M.; Timazhev, A. V. (2019): Time lag between changes in global temperature and atmospheric CO2 content under anthropogenic emissions of CO2 and CH4 into the atmosphere. In IOP Conf. Ser.: Earth Environ. Sci. 231, p. 12039. DOI: 10.1088/1755-1315/231/1/012039.

Pedro, Joel B.; Jochum, Markus; Buizert, Christo; He, Feng; Barker, Stephen; Rasmussen, Sune O. (2018): Beyond the bipolar seesaw: Toward a process understanding of interhemispheric coupling. In Quaternary Science Reviews 192, pp. 27–46. DOI: 10.1016/j.quascirev.2018.05.005.

The current climate change is caused by the consistently increasing greenhouse effect, which results from increasing greenhouse gas concentrations in the atmosphere. These greenhouse gases (carbon monoxide, carbon dioxide, methane, ozone and nitrous oxide) derive mainly from anthropogenic sources, like the burning of fossil fuels, deforestation, agriculture, and the production of cements.

Since the industrial revolution, the increasing emission of greenhouse gases led to a significant increase in the global greenhouse effect. And by continuing these emissions, the greenhouse effect is more and more amplified. Observations throughout the world make it clear that climate change is occurring, and scientific research concludes that the greenhouse gases emitted by human activities are the primary driver. This conclusion is based on multiple independent lines of evidence and the vast body of peer-reviewed science. The majority of actively publishing scientists (97%) agree that humans are causing global warming and climate change.

Further information

The simplest way is to measure the concentration of global greenhouse gases and temperature over the years and correlate the data with historical data and the introduction of anthropogenic greenhouse gas sources. These documented increases in temperature can also be correlated with exact atmospheric compositions, which can be drawn from ice core records.

The global mean temperature has been well-documented since 1750. From the industrial revolution onwards, which initiated the large-scale emission of greenhouse gases through the burning of fossil fuels, a more rapid temperature increase is observed. These combustion-produced CO2 emissions are a special case of the greenhouse effect, the so called Callendar effect. A worldwide average surface temperature increase of around 1°C has been documented since 1880, relative to the mid-20th-century baseline (of 1951 – 1980). This is on top of an additional 0.15 °C of warming from between 1750 and 1880.

Nowadays satellites are able to measure the energy radiation, that is reflected by the Earth back into space. These observations are an indirect proxy for the concentration of greenhouse gases in the atmosphere. The measurements document that with increasing concentration of greenhouse gases, the percentage of reflected radiation decreases. This decrease can be associated to the wavelengths of carbon dioxide, methane and ozone, all of which are emitted into the atmosphere through anthropogenic sources.

Ozone (O3) is a greenhouse gas in the stratospheric level, between 10 and 50 km above Earth’s surface. Ozone has a concentration of 2 – 8 ppm, while the remaining 210,000 ppm are taken up by oxygen (O2). Ozone in the upper atmosphere absorbs short-wave ultraviolet (UV) rays between 240 and 160 nm. This high energy radiation can produce ozone from oxygen, also known as photochemical reaction. The process of ozone creation and destruction is called Chapman cycle. Although ozone is created primarily at tropical latitudes, large-scale air circulation cells in the lower stratosphere move ozone towards the poles, where its concentration increases.

The ozone in the stratosphere absorbs a large part of the solar high energy radiation, which is harmful to organisms. The UV radiation can change chemical structure of molecules due to its high energy and cause gene mutations. The stratospheric level acts like a barrier for the dangerous radiation emitted by the sun. Ozone can also be found at ground level, which is a human health irritant and component of smog. This tropospheric ozone is produced by anthropogenic sources and has nothing to do with the “ozone hole”.

The term “ozone hole” refers to the depletion of the protective ozone layer in the stratosphere over Earth’s polar regions (see figure below). The concentration of ozone in this area is lower, which causes more high-energy radiation to pass. This increase in UV-radiation causes harm to the organisms there, resulting in health problems, from eye damage to skin cancer.

Why is ozone depleting?

The thinning of the ozone layer is caused by increasing concentrations of ozone depleting chemicals – chlorofluorocarbons (CFCs) and, to a lesser extent, halons. These chemicals can remain in the atmosphere for decades to over a century. These gas molecules were found in refrigerators or aerosol-cans, but their use was prohibited with the Montreal protocol in the 1980s. During the dark polar winters, stratospheric clouds form due to very low temperatures. These clouds last until spring and create the condition for drastic ozone destruction, since they provide a surface for chlorine to change into its ozone destroying form.

The CFCs attach to the polar stratospheric clouds until they are released in spring. When released, these ozone destroying molecules attach to the ozone and break their molecular bonds. After this process, the molecular structure of ozone is altered and the ability to absorb UV radiation is reduced. Due to the CFC molecules, the concentration of ozone at the poles has decreased significantly and ultimately resulted in a “hole”. The ozone reduction in the Arctic is comparable to that in the Antarctic. Ozone can also be removed by aerosols emitted from non-anthropogenic sources. Volcanic eruptions can emit sulfate particles into the stratosphere, which have a similar effect to the CFCs.

Figure 5: False-color view of total ozone over the Antarctic pole. Purple and blue represent areas where there is the least ozone, yellows and reds where there is more ozone. Source: NASA Ozone Hole Watch.

Does the ozone hole affect climate change?

The continuously increasing greenhouse effect prevents heat from the lower atmosphere to traverse into the stratosphere. This effect occurs due to an increasing amount of greenhouse gases, which absorb the heat radiation in the troposphere and reflect it back to the surface. The troposphere then acts like a heat barrier, preventing the heat from rising to the stratosphere, which in response is cooling down. In a cooler stratosphere, ozone loss creates a cooling effect that results in further ozone depletion. UV radiation releases heat into the stratosphere when it reacts with ozone. With less ozone, there is less heat released, amplifying the cooling in the lower stratosphere and enhancing the formation of ozone-depleting polar stratospheric clouds, especially near the South Pole.

With less ozone in the stratosphere, the more UV radiation passes through the stratosphere to the surface. However, UV radiation plays only a small role in global warming, because the concentrations in the solar radiation are too low to contribute to a significant warming effect. The net effect of UV-radiation is to cool the stratosphere rather than warming the troposphere. Therefore, the ozone hole does not contribute to the effect of global warming. Yet the depletion of ozone is problematic, since the increasing amount of UV-radiation may affect the health of organisms.

What scientists have uncovered recently, however, is that the ozone hole has been affecting climate in the Southern Hemisphere. That’s because ozone is also a powerful greenhouse gas and destroying it has made the stratosphere over the Southern Hemisphere colder. The colder stratosphere has resulted in faster winds near the pole, which somewhat surprisingly can have impacts all the way to the equator, affecting tropical circulation and rainfall at lower latitudes. The ozone hole is not causing global warming, but it is affecting atmospheric circulation.





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