This little number is on everybody’s mind as negotiators from more than 190 countries gather in Paris to seek an agreement to reduce global carbon emissions enough so that the two-degree threshold is not passed.
The two-degree increase in average temperature has been a generally accepted climate target for a long time, based partly on science and partly on a political decision to create a focal point. It seems to stem all the way back to 1975 when Yale economist William Nordhaus introduced the idea that an increase in temperature of two to three degrees would bring the climate outside of its “normal” range. A growing body of research supported this and two degrees became a policy goal for many nations. Among other things, it was part of the Copenhagen Accord from the Climate Change Conference in Copenhagen in 2009, which states that “…recognising the scientific view that the increase in global temperature should be below two degrees C”.
Vattenfall R&D meteorologist Jesper Nielsen Nissen says it is no longer up for debate whether or not man is responsible for the increase in global temperature. And in his eyes, the energy and heat sectors that contribute 25% of all carbon emissions have a unique opportunity to act and to contribute to the world staying below the safe threshold of two degrees.
Every degree matters
2015 has been the hottest year on record and the world has now reached the halfway point to a 2 degree increase. Opinions about how much warming the Earth can take are numerous, so setting one common temperature increase target will always to be a matter of a trade-off between a range of factors.
According to a report from the US National Academies’ National Research Council, it is now possible to use change in globally averaged temperature degree by degree as a tool for examining a wide range of climate impacts. Among other things, the report states that each degree C of global temperature rise can be expected to produce a 3-10% increase in the amount of rainfall during the heaviest precipitation events and a 200-400% increase in the area burned by wildfire in parts of the western United States. And according to the Nobel Peace Prize winning UN Intergovernmental Panel on Climate Change, IPCC, the Arctic sea ice extent has shrunk by 0.51 million square km in every successive decade since 1979 – an area that is actually a little larger than the size of Spain.
In other words, each degree humans warm up the globe is likely to bring us closer to serious and irreversible changes in the climate.
Climate models - the scientists’ crystal balls
To help get a clear picture of the situation, scientists use detailed climate models to predict the complex climate system many years into the future and describe the consequences of continued human emission of CO2.
“Climate models have proved very good at describing the temperature increase that has been seen in the past 150 years during industrialisation and can therefore also be trusted when looking at the future climate. This means that their predictions are useful tools in guiding the political process as to establish a ‘carbon budget’”, Nissen explains.
“There will, however, always be uncertainties in even the best climate models. Some will be related to human activity, such as how we use the land and how we produce energy in the coming years, and some will relate to unknown physical processes in the complex climate system. For instance, scientists cannot tell with 100% certainty where the so-called “tipping points” are, where things get into a self-fuelling spiral. What happens if both ice caps melt and heat is no longer reflected by the white ice but instead absorbed by the dark water? And what happens if the permafrost in Russia melts and releases a lot of methane into the atmosphere? And, although we know that some 30% of man-made carbon dioxide is absorbed by the oceans, we do not know for sure whether that will still be the case at higher average temperatures”.
Ten years to go
According to Nissen, scientists are working with so-called “climate sensitivity” figures that for instance tell us how much warming will occur if we double the CO2 content in the atmosphere compared to preindustrial levels in 1880. IPCC estimates that we have a 2/3 chance of staying below two degrees if we can limit total emissions to 1,000 gigatonnes of carbon (GTC) – but we have already burnt more than 515 GTC since preindustrial times.
“If we continue with the current annual burn rate of 50 GTC, the world will have ten years to go before we reach a warming of as much as two degrees, according to IPCC. And this is a conservative estimate – other scientists expect a substantially higher amount of warming for the same amount of carbon. This huge range in expected warming illuminates the underlying uncertainty in estimating the remaining available carbon budget” Nissen warns.
Nissen concludes that: “With all these uncertainties, it seems that scientists and politicians have found two degrees to be a generally accepted target to work towards and it is now up to COP21 to deliver on all the nice words.”
Information about COP21 and Vattenfall at COP21 and the Sustainable Innovation Forum
IPCC Summary for policymakers: Assessment report 5 2014
Warming world, impacts by degree (pdf)
BBC special report on climate change
CNN on 2 degrees