The two degree target is the international climate policy goal of limiting global warming to less than two degrees Celsius compared to pre-industrialization levels (1850–1900). It is an integral part of the Paris climate agreement.[1] This objective is a political determination based on scientific knowledge concerning the probable consequences of global warming, which dates from the Copenhagen Conference in 2009.[2] Physical climate risk scenarios, which often project to the end of the century, 2100, use the 2° target as a reference point. The time at which global mean temperature is predicted to reach +2°C compared to the pre-industrial period (1850–1900) is termed the “crossing year".[3]
As a global target for limiting emissions, the 2° target has frequently been criticized for being higher than desirable,[1][4] because two degrees of warming will have serious consequences for humans and the environment.[5] The IPCC Special Report on Global Warming of 1.5 °C (2018) included detailed analysis of the probable differences in impact of "limiting global warming to 1.5 °C compared with 2 °C", warning that a 2 °C temperature increase would worsen impacts that include extreme weather, Arctic sea ice decline, rising sea levels, coral bleaching, and ecosystem loss.[6]
The impact of climate change is not uniform:[7] for example, land regions tend to warm faster than ocean regions.[6] NASA has modeled predicted changes in six key climate variables: air temperature, precipitation, relative humidity, wind speed, and short- and longwave solar radiation, with particular attention to their impacts on heat stress and fire weather. NASA's NEX-GDDP-CMIP6 data set models impact at a fine-grained spatial scale, which can be used to identify key risk areas and develop adaptation and mitigation action plans for specific regions. Above the 2°C threshold, dangerous and cascading effects are predicted to occur, with many areas experiencing simultaneous multiple impacts due to climate change.[7][8]
As of 2022, the UN Environment Programme reported that countries have not met their climate goals to date. As a result the UN Framework Convention on Climate Change predicts a likely increase between 2.1°C and 2.9°C in temperature by 2100, exceeding the 2°C climate target.[9] Some scientists suggest that the development of decarbonization technologies may offer a way to reverse the accumulation of CO2 in the atmosphere. Even if temperatures increase above 2°C, it may become possible to halt or reverse increases by late in the century and bring CO2 levels back to the levels identified by the Paris climate agreement.[3] This type of scenario is referred to as an "overshoot pathway".[6] Achieving such an outcome will require multigenerational management over many decades.[3]
