Since Copenhagen (2009) Accord under the UN climate negotiations we have 2 policy options in the table, the under 1.5C as well as the longstanding (1996) 2C limit.
The Carbon Brief's 'Two degrees: The History of climate changes speed limit' is a good short background to the 2C policy target.
The 2015 UN Paris Agreement Article 2 objective is 'Holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change'
"Global warming of 2ºC above pre-industrial levels cannot be considered safe. Considerable climate change impacts are already felt today and will have to be faced in the future – even below 2°C"
(The 2C Target EU Climate Change Expert Group ‘Expert Group Science’ July 2008).
A global warming limit of 2°C has become an almost universally accepted "target," aimed for by national and international policy.
The 2014 IPCC AR5 assessment clearly shows that
The 2 degree target is disastrously high according to James Hansen, and we can see now with accelerating Arctic changes at today's 1.0C warming (see below) is an invitation to global climate catastrophe.
James Hansen has been published explaining from our current knowledge of the science 1C is the danger limit. The best paper easy read on 1C is The Case for Young People and Nature by Hansen and a dozen other experts in different fields of research related to global climate change effects.
The 1C danger limit is clear from the impacts on crop yields (see food page).
Also, the IPCC AR4 has crop losses to small holders and subsistence farmers and serious health impacts stars below 1.0C and re not being recorded.
The IPCC AR4 has population health impacts starting below 1.0C.
Arctic global warming changes at today's 1.0C:
And certainly a 450 parts per million (ppm) target for carbon dioxide is too high even to have a high certainty of staying below the 2 degree target. James Hansen has published that we must get below 350ppm from today's almost 400ppm.
We must also get below 350ppm CO2 to prevent catastrophic ocean warming and acidification. With respect to the all important atmospheric greenhouse gas concentration, anything above 350 ppm is a formula for a catastrophic temperature increase above 2 degrees.
Climate safety, therefore, is below 350 ppm (James Hansen et al, 2008) and is probably under 300 ppm, which has been the upper limit for atmospheric carbon dioxide over the past 800,000 years (according to the ice core data). To achieve climate safety, we might even need to get back down to the pre-industrial CO2 level of 280 ppm (Hans Schellnhuber, 2008).
The oft stated goal to keep global warming less than 2ºC is a recipe for global disaster, not salvation.
— Dr. James Hansen, 2008
"The risks for worst-case outcomes amplify much more quickly than the risks for most likely outcomes. For an early and rapid decline in emissions, the worst-case outcome is around 0.7ºC higher than the most likely temperature rise. With much slower action taken much later, the difference between the most likely and worst-case outcome is almost twice as wide, at 1.2ºC. This takes a worst-case temperature rise of less than 3ºC to one just above 5ºC by the end of this century, bringing with it significant risk of dangerous impacts to our environment, society and economy.
A major reason for this amplification is the so-called 'carbon cycle effect'. Plants, soils and oceans currently absorb about half of the carbon dioxide emitted by human activities, limiting rises in atmospheric CO2 and slowing global warming. As temperatures increase, this absorption is very likely to decrease. For example, plant matter in the soil breaks down more quickly at higher temperatures, releasing carbon more quickly, and amplifying the warming trend. Methane released from the thawing of permafrost will add to the warming. This methane release is currently not included in the calculations, and becomes more of a risk for larger temperature rises.
Hence, the risks of dangerous climate change will not increase slowly as greenhouse gases increase. Rather, the risks will multiply if we do not reduce emissions fast enough."
— Dr. Vicky Pope, Head of Climate Change Advice, Hadley Centre, UK Met Office, October 2008
The 2ºC "target" is a politically (not scientifically) set target from the European Union (EU) that goes back to the mid 1990s.
Since then, although the weight of scientific evidence has increasingly shown that a globally averaged 2ºC temperature increase will be disastrous for humanity and much of life on Earth, the figure has stuck.
A safe limit was established at +1ºC even before 1990 (Villach Conference, 1987). The Avoiding Dangerous Climate Change conference in Exeter (2005) arrived at a limit of 1ºC (although acknowledging that dangerous climate change for developing countries is below 1ºC).
Many papers have been published over the years saying that 2ºC is too dangerous, and the precautionary weight of evidence is 1ºC to 1.5ºC. (These figures are now superseded by actual changes in the Arctic.)
The EU has always acknowledges that a 2ºC global warming is not safe and is a policy compromise hoping to prevent irreversible planetary impacts.
"[...] overall global annual mean surface temperature increase should not exceed 2°C above pre-industrial levels in order to limit high risks, including irreversible impacts of climate change; RECOGNISES that 2°C would already imply significant impacts on ecosystems and water resources. [...]" (2610th Council Meeting, Luxembourg, 14 October 2004)
At a 2005 meeting in Buenos Aires, the European Union addressed the question of what is dangerous climate change under the United Nations Framework Convention on Climate Change (UNFCCC). They came to the conclusion that dangerous global warming lies between 1.0ºC to 1.5ºC, thereby giving a safe limit of 1ºC.
The United Nations Environment Program (UNEP) has published a danger level of 1.5ºC based on 2007 IPCC data.
But these new figures have not been noticed, and the 2 degree target remains virtually the only limit cited in mass media and many scientific media.
Video Dr. H. Montgomery lecture says it all
Lancet Countdown (Nov)2018 Report for policy makers
2017 WHO 5th Climate & Health Summit