Halving Carbon Emissions to Prevent Some Warming

A new study reports that halving carbon emissions from 1990 levels by 2050 would likely be enough to keep total warming below 2°C.

“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.

This provides some additional scientific support for the widely cited political goal of reducing carbon emissions 80% by 2050. Since an increase of 2°C is often used as a shorthand for the maximum temperature increase that will not trigger catastrophic warming, this is good news. Preventing the worst may be possible. Further decreases beyond this study's 50% may have further salutary effects, such as preserving Arctic sea ice. Unfortunately, the news is not as good as it may seem. First, we have used up a lot of our time already.

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.

That concentration has continued to increase over the past four years.

Second, a world with 2°C of warming is not necessarily going to be a pleasant place. True, it ought to avoid the worst effects, but even that small amount could force some major changes in world and local climates. In the Antarctic Peninsula, where the temperature has been rising much faster than the global average, this could reduce or eliminate some penguin species. Other species would presumably undergo similar declines or shifts in range.

According to the most recent IPCC report, a global average temperature rise of 2°C would still result in a sea level rise of 7-14 inches (18-38 cm) by 2100, in other words roughly the same increase as occurred in the 20th century. While smaller than the worst scenarios, such a rise would still imperil coastal wetlands through loss of subaquatic vegetation and changes in salinity. Note also that many of the effects of climate change on people start on the lower end of the scale, the increases of a degree or two, and build as the temperature rises further. This means at even 1-2° temperature rise we may see changes in our agriculture, water supply, health, and more.

The scenario presented in this article is not exactly rosy, but it may be the best we can hope for under current circumstances. Better yet, it may actually be achievable.

Update: Of course, it would be better to do this sooner rather than later.

Further update: Here is another take on the same articles.