A new paper in Nature looked at the probability of keeping average global temperatures from rising more than 2°C above pre-industrial levels under varying levels of climate policy stringency - basically, mitigation costs. It also sought to quantify and rank the uncertainties associated with efforts to mitigate climate change, including questions about the climate itself, uncertainties related to future technologies and energy demand, and political uncertainties as to when action will be taken.
The climate system itself is full of uncertainty – an oft-used argument to postpone climate action until we have learned more. The difficulty of accurate numerical models only adds to the confusion and political bias by academics tops off concern that global warming may be as much hype as harm.
The most important uncertainty is political – that is, the question of when countries will begin to take serious action to reduce greenhouse gas emissions and implement other policies that could help mitigate climate change. The United States, for example, has drastically reduced its CO2 emissions, dropping to early 1990s levels overall and to early 1980s levels from energy like coal. But other countries that are exempt from current political agreements, like China, India and Mexico, more than made up for the USA.
Social uncertainties, which influence consumer energy demand, were second-most important, the study found. Social uncertainties refer to things like people's awareness and choices with respect to energy and to the adoption of efficient technologies.
The researchers examined geophysical and technological uncertainties and found that while the climate system and energy supply technologies are generally seen as the major factors for climate, they ranked below political and social uncertainties in the new study. Geophysical uncertainties refer to the unknown – and unknowable – factors about how the climate system will react to greenhouse gas emissions. Technological uncertainties refer to questions about which energy supply and carbon capture systems will be available in the future.
The authors used scenarios to define how these factors affect the probability of staying within a given temperature target, at a variety of carbon prices. In addition to the 2°C target, the researchers also explored the distribution of costs and risks for limiting global warming to below 1.5°C and 3°C.
Even for a 3°C target, a 20-year delay in the most stringent greenhouse gas reductions in combination with a high demand future means that there would remain a one in three chance that temperatures exceed 3°C.
At the same time, limiting warming to 1.5°C with at least a fifty-fifty chance (a target supported by the least-developed countries and small island states) appears only to be possible if the world starts acting on climate change now and turns towards an energy-efficient future.
"We wanted to frame the problem in a new way and try to understand which uncertainties matter in trying to limit global warming by specific climate action," says Joeri Rogelj, ETH researcher and lead author on the paper, who carried out the research at IIASA.
Keywan Riahi, IIASA energy program leader and study co-author says, "With a twenty-year delay, you can throw as much money as you have at the problem, and the best outcome you can get is a fifty-fifty chance of keeping temperature rise below two degrees."
Two degrees is the level that is currently supported by over 190 countries as a limit to avoid dangerous climate change.
"How much energy the world consumes going forward turns out to be a much bigger swing factor for climate change than the availability of technologies like solar and wind power, biofuels, and so on," said IIASA researcher David McCollum, another co-author. "Energy efficiency, improved urban planning, lifestyle changes – these things on the demand side of the energy equation are so important; yet they receive relatively little attention compared to the supply side."
Surprisingly, while much research is focused on understanding the global climate, a highly complex system with many uncertainties, the new study finds that after a certain point, there is little chance of limiting temperature rise to below 2°C.
"Ultimately, the geophysical laws of the Earth system and its uncertainties dictate what global temperature rise to expect," said Rogelj. "If we delay for twenty years, the likelihood of limiting temperature rise to two degrees becomes so small that the geophysical uncertainties don't play a role anymore."