Climate change: it’s only intermission
Global warming continues, even if the worst-case prognosis has become slightly less probable
Global warming again and again poses riddles for climatologists - but one thing is almost certain: in the next decades, the average temperature on the Earth will continue to increase, even though it increased much more slowly from 2001 to 2010 than during the previous decade. This is supported by new predictions from an international research team headed by scientists from the University of Oxford and under the participation of Directors Jochem Marotzke and Björn Stevens of the Max Planck Institute for Meteorology. Based on current climactic data, the scientists have re-calculated how much the air at the Earth’s surface will have warmed once the carbon dioxide level of the atmosphere has doubled. This will be the case approximately mid-century if the concentration of greenhouse gases continues to grow at or above the current rate. The average temperature will have increased then by 0.9 to 2.0 degrees Celsius compared to pre-industrial levels. Through to the end of the century, the atmosphere would warm up considerably more than the two degrees which the UN Climate Change Conference wants to limit this warming to.
Climate change has been variable over the past two decades. While the global average temperature climbed faster than ever before during the 1990s, namely 0.24 degrees Celsius, it only increased approximately 0.03 degrees Celsius during the subsequent decade. “We have not been able to explain this weakening of the temperature increase at the Earth’s surface with our models up to now,” says Jochem Marotzke. “Nevertheless, the Earth has warmed overall. However, this warming has taken place primarily in the deeper layers of the oceans.”
Jochem Marotzke is part of a team of the most renowned climatologists in the world that is now taking into account the most recent increase of surface temperatures in a new prediction of how the Earth is warming due to the effects of greenhouse gases, primarily through carbon dioxide (CO2). This prognosis confirms that the climate models correctly predict the trend in global warming over a period of several decades, to about the middle or end of the 21st century. As a result, there are no grounds for an all-clear signal.
Following a doubling of CO2, the ultimate warming will take place over hundreds of years
The team working with Alexander Otto and Myles R. Allen from the University of Oxford differentiates between the medium-term and long-term reaction of the climate to a doubling of carbon dioxide content in the air expected to be reached in 2050. The greenhouse effects of the gases caused by this become immediately noticeable as soon as the carbon dioxide concentration has reached this level. The extent is expressed by climatologists as the transient climate response (TCR).
Since the climate system possesses a lot of inertia and the oceans warm up only very slowly for instance, it takes a while until the effect of the greenhouse gases fully develops: warming via the greenhouse gases is amplified through numerous feedback loops, but also weakened by several processes. Only once this complicated interaction has settled does the climate attain a stable state again. Climatologists calculate this long-term reaction of climate as the equilibrium climate sensitivity, or ECS. This corresponds to the ultimate temperature increase resulting from a doubling of the CO2 concentration that presumably ensues only after several hundred years.
The medium-term climate response as well as the long-term reaction reveals something about the strength of the feedback loops between the CO2 rise and global warming. The international team has now re-calculated both values.
Besides the temperature increase measurements during the past decade, there are important factors that go into the calculation that are critical to the Earth’s thermal budget. The most important one is the solar energy that irradiates Earth. This also includes, however, the heat that cannot be re-radiated back out into space due to the greenhouse gas effects of carbon dioxide. This heat amounts to almost exactly 3.44 W per square metre for a doubling of the CO2 concentration. In addition, the effects of volcanic eruptions and aerosols are introduced into the calculations. In the latter case, these are particles suspended in the air that shield against solar radiation on the one hand, and serve as condensation nuclei for cloud droplets on the other. The long-term reaction of climate additionally takes into account the oceans’ uptake of heat over time.
Warming of far more than two degrees toward the end of the century is a threat
Using these values, the researchers have calculated that the atmosphere close to the Earth’s surface will have warmed by 0.9 to 2.0 degrees Celsius with 90 per cent probability for a doubling of the CO2 content. The most probable amount is a temperature increase of 1.3 degrees. “The transient climate response we have calculated using the most current measurements lies within the bounds of climate model predictions, if not at its upper boundary,” says Alexander Otto, who performed the calculations at the University of Oxford.
If no additional greenhouse gas was emitted into the atmosphere following a doubling of the carbon dioxide concentration, the Earth would warm up about 1.2 to 3.9 degrees compared with the pre-industrial values with 90 per cent certainty during the following centuries. The most probable long-term reaction of the climate is a rise of about two degrees. “How high the long-term warming will turn out is still quite uncertain, however,” says Otto. “But for most of the political decisions, it is in any case critically important just how strongly the warming will actually be over the next 50 to 100 years.”
The Earth may therefore not heat up as much as might be feared from the worst-case prognosis. “That is no doubt good news,” says Reto Knutti from the Swiss Federal Institute of Technology of Zurich and one of the participating researchers. “However, if the greenhouse gas emissions continue without respite, we will nevertheless have a temperature increase of much more than two degrees at the end of the century.”
How aerosols and clouds are involved with climate is not completely clear
Whether the Earth actually does warm somewhat slower, as many climate models have thus far suggested, remains uncertain – despite the measurements since 2000. “In view of what we know and don’t know about climate variability, we shouldn’t read too much into a single decade,” says Jochem Marotzke. This is because the researchers still need to clarify several details about how climate reacts to the increase of greenhouse gases. “At the moment, we assume for example that the effect of the feedback loops remains constant over time”, says Jochem Marotzke. “But we don’t know whether that is actually true.”
In addition, the role of aerosols is uncertain. How much sunlight do particles suspended in the upper atmospheric layers reflect? And how do they influence the formation of clouds and precipitation? Speaking of clouds. They are involved with climate in numerous ways: they do not just bring rain, they also shield against sunlight. But it is not clear how they react to global warming. Do more clouds form when the Earth becomes warmer, because more water evaporates then? Or do fewer clouds form because the air currents change?
Many questions therefore remain unanswered. But the knowledge gaps are closing. “Climatology is especially interesting at the moment,” says Björn Stevens. “The measurements of global heat exchange and the composition of the atmosphere have made enormous progress in the last two decades,” he says. “Since we’ve made further improvements to the models and can test them better, we are making rapid progress, especially regarding how the Earth will react to the rise in greenhouse gases.”