The amount of carbon dioxide (CO2) in the atmosphere is rising and is now higher than at any time in the last 3 million years. This increase in CO2 is entirely caused by human activities, mostly the burning of fossil fuels.
CO2 is a potent heat-trapping “greenhouse gas.” Scientists first discovered the heat-trapping properties of CO2 in the 19th Century, and they have been concerned about global warming due to rising CO2 levels for many decades.
Our atmosphere, ocean waters and land surfaces are all warming. More than 90% of the heat that has accumulated from global warming in recent decades is stored in the oceans, due to their great capacity to take up and store heat.
Measured increase in heat stored in different depth layers of the ocean, compared to the heat trapped by greenhouse gases (TOA means top of the atmosphere radiation imbalance). Times of major volcanic eruptions are marked near the bottom of the graph. Source: Cheng et al. 2017.
Many studies have estimated how much of the global warming that has occurred since the mid-20th Century is due to human activities versus natural contributions. The conclusion of these studies is that human activities have caused slightly more than 100% of the observed warming. In other words, without the human activies, global temperatures would have cooled slightly since the mid 20th Century as a result of natural factors.
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Prof. Stefan Rahmstorf
Prof. Edward Maibach