How the “2°C target” can be reached
If CO2 emissions are halved by 2050 compared to 1990, global warming can be stabilised below two degrees. This is shown by two studies by a co-operation of German, Swiss and British researchers in the magazine “Nature”.
To contain global warming, and its risks and consequences, warming compared to pre-industrial times (pre 1900) should not exceed two degrees Celsius. Although, according to the reports of the Intergovernmental Panel on Climate Change (IPCC), there is no specific temperature threshold for dangerous climate changes, and the negative effects are gradually increasing, over one hundred countries have adopted this “2°C target”. Scientists have used a new probability model to calculate how much CO2 our atmosphere tolerates under these target specifications. This1 and another study2, today published in “Nature”, produced similar results: From 2000 to 2050, a maximum of 1000 billion tonnes of CO2 may be emitted into the atmosphere. Roughly speaking, today, around one third of this wad has already been shot.
Like a bath tub
“The behaviour of CO2 in the atmosphere is best described as a full bathtub,” says Reo Knutti, professor at the Institute for Atmosphere and Climate at ETH Zurich, and co-author of one of the two studies. The inflow of the bathtub is large, but the drainage is small. The CO2 emissions are increasing every year, but the CO2 is only removed from the atmosphere very slowly. To not let the bathtub overflow, the inflow must thus be stopped early enough. “It is wrong to believe that the temperature will remain constant with constant emissions,” says Knutti.
The innovative aspect of the study is the fact that the probabilistic model does not perform just one individual simulation, but simulates thousands of combinations of scenarios and assumptions. Knutti adds that, in doing so, all known uncertainties are taken into account. For example, the physical uncertainties in feedback effects from clouds, uncertainties in the carbon cycle – e.g. how much CO2 is absorbed by the oceans -, as well as uncertainties in the scenarios. These scenarios describe the time when the maximum “allowed” emission has been reached. The model also includes the overall effects of all greenhouse gases, such as CO2, methane and nitrous oxide, as well as ozone and the aerosol effect.
Everything in one step
So far, these sorts of studies have been conducted in two stages, Knutti explains: “First, the 2°C temperature threshold is converted into a CO2 target, and then the carbon model is used to calculate how high the emissions can be.” The new study now does this in one step. To do this, scientists have redeveloped and adjusted existing models. They calibrated the calculations on the observed data from the last hundred and fifty years.
Budget quickly exhausted
The models show that there is a 75 percent probability that global warming will not exceed two degrees if a maximum of 1000 billion tonnes of CO2 are emitted into the atmosphere from 2000 to 2050. This number seems high, but 234 billion tonnes had already been flung into the atmosphere between 2000 and 2006. If the emission remain at this high level, or even increase, the budget would be exhausted before 2030. The results show that time to act is short. Knutti is pleased that greenhouse gas emissions in Switzerland in 2007 were 2.7 percent lower than in 1990 but the values continue to be too high: at least a 50 percent reduction is needed worldwide by 2050; the global long-term goal would be less than one tonne per person per year. Currently, some 6 tonnes of CO2 per person are emitted in Western Europe each year, 19 tonnes in North America and 3 tonnes in China – without taking into account grey energy.
The researchers believe that the next forty years until 2050, in which, according to the study, CO2 emissions must be halved, will be a good indicator of how global warming will develop. “If, in 2050, we find that our measures have been successful and are working, and if we continue to implement them, we can assume that we are on the right track,” says Knutti.
The study also shows that, if all conservatively estimated available fossil fuels were to be burnt, two to three times more CO2 than allowed for the 2°C target would be emitted. This only takes into account the fuels which are already known and which are economically viable to extract. The fossil fuels will therefore not run out before the maximum CO2 emission calculated by scientists is reached. If we continue to use them, this must take place in combination with effective technologies which capture the CO2 and extract it from the atmosphere. Whether such technologies can be implemented on such a large scale and at sensible prices is, however, questionable.
“Every tonne of CO2 is one tonne”
Together with the aforementioned study, Nature also included a second publication on a similar issue, which, however, takes into account a longer period of five hundred years, until the year 2500. This study also concludes that the total amount of CO2 emissions is crucial in terms of how much the earth warms up. The authors summarise a political interpretation in comments in Nature Reports Climate Change3. According to Knutti, “Every tonne of CO2 is one tonne, whether it is emitted today or in fifty years. This is often lost in the tangle of emission targets, certificates and negotiations. The total quantity is what matters, and must be limited, but short-term goals are necessary to see whether we are on the right track.”
Knutti has also recently shown that, in a complex climate model and in co-operation with other scientists, with a 70 percent CO2 reduction by the end of this century, the melting of the Arctic Sea ice can be limited to a quarter4. The researchers conclude that this could, for example, help to protect the fauna – such as the polar bears – and prevent the permafrost areas from decreasing by 70 percent. Instead, “only” 45 percent of the permafrost are expected to melt. The frozen permafrost soils are natural reservoirs of greenhouse gas. They store large quantities of CO2 and methane, which can be emitted into the atmosphere if the soil melts.
The series of studies show that the total quantity of CO2 emission is limited if people want to limit climate change. “With every year of delay, we are using up our quota, losing flexibility, and increasing the probability of dangerous consequences,” says Knutti.
1Meinshausen et al.: Greenhouse gas emission targets for limiting global warming to 2°C, Nature 2009, 458, 1158-1163, doi:10.1038/nature08017
2 Allen et al.: Warming caused by cumulative carbon emission: the trillionth tone, Nature 2009, 458, 1163-1166 doi:10.1038/nature08019
3 Allen et al.: The exit strategy: Emission targets must be placed in the context of a cumulative carbon budget if we are to avoid dangerous climate change. Nature Reports Climate Change, doi:10.1038/climate.2009.38 (2009).
4 Washington et al.: How much climate change can be avoided by mitigation? Geophysical Research Letters 2009, 36, L08703, doi:10.1029/2008GL037074