6. How do we know humans are causing climate change?
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.
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.