Climate change – facts, consequences, risks

“Climate change – facts, consequences, risks”

Prof. Dr. Anders Levermann, Potsdam Institute for Climate Impact Research, Germany

Background information on climate change

The concentrations of carbon dioxide, methane and nitrogen oxides in the atmosphere are rising rapidly.  This rise has been caused by anthropogenic emissions.  The absorption of carbon dioxide by terrestrial vegetation and the ocean reduces the amount of CO2 left in the atmosphere by approximately 46%.  The absorption spectra and therefore also the greenhouse impact of these molecules are known from basic physical calculations and laboratory experiments.  The prevailing positive climate feedbacks strengthen the direct radiation effect and lead to a “climate sensitivity” of 3±1 °C.  The global warming is the result of the doubling of pre-industrial carbon dioxide concentration from 280 ppm to 560 ppm.  This knowledge is undisputed amongst climate researchers, it is independent from model simulations, and it is based on fundamental physical equations.  This is the reason for future considerable warming as a consequence of further greenhouse gas emissions.

There is a question that should be looked at separately, that of whether we have already changed the climate.  Today, the CO2 concentration is 380 ppm; if we add the other anthropogenic greenhouse gases to this measurement, the result is about 420 ppm of CO2 equivalents.  The temperature growth of approximately 0.8 °C globally that we saw in the last century is consistent with this atmospheric composition, particularly with the cooling effect of the pollution by aerosols, which partly covers up the greenhouse effect.  None of the natural factors that influence the global average temperature (volcanoes, the sun’s rays, cosmic radiation, etc.) has shown a comparable consistent trend in the past 60 years (at minimum).  If in addition to the natural influences anthropogenic greenhouse gases are also taken into consideration, climate models can simulate the development of the last century; without considering anthropogenic greenhouse gases, the models cannot produce the simulation. Considering the temperature fluctuation of the last 1 000 years, this seemingly low warming of 0.8 °C is unusual enough.

Already observed consequences

Although the warming signal of the last 100 years rises relatively slowly from the background of uncertainty and natural fluctuations, the consequences of the climate change have been already observed.  One of them is global mountain glacier melting, as well as the rise of the sea level by 15 cm to 20 cm within the last century.  The Arctic sea ice shows a dramatic decline based on a self-reinforcing feedback process: As the light-colored ice melts, its reveals more sea surface, which is darker in color.  As a consequence, less of the sun’s radiation is reflected back into the atmosphere, but is instead absorbed.  The resulting warming strengthens the ice melting (ice-albedo feedback).  Such self-reinforcing feedback processes play a special role in the climate system because they can lead to very drastic reactions of some partial systems to even small disorders.  This is a very important fact, particularly in relation to possible future risks in a world that is becoming warmer.

Future projections

In comparison with the future temperature projections for the next century, when we review fluctuations in the past, the reconstruction uncertainty as well as the already observed growth become smaller.  The so-called “business as usual” scenario, in which the rise of emissions is similar to growth in the past, shows a rise of approximately 4° C compared to the pre-industrial era (in the average of all the models). The main uncertainty for future development does not result from a model uncertainty, but from the uncertainty of our socio-economic and political activities.

In 1996, the EU formulated a political goal which said that in order to avoid “dangerous climate change”, the global temperature rise must be kept under 2° C compared to the pre-industrial time.  In order to achieve this goal, global greenhouse gas emissions must be reduced by at least 50 % by 2050 compared to 1990.  Due to the uncertainties in our current perception of climate change, it is possible that 40 % reduction might be enough, but it is also possible that we need a 70 % reduction. Since the climate system cannot absorb any further carbon dioxide, the emissions must in fact disappear completely after 2050.  None of the scenarios in figure 3 implies political avoidance strategies.  However, the projections show us that they are necessary in order to achieve the EU goal of 2° C and in order to, for example, prevent the complete melting of the Greenland ice sheet.

Risks in a warmer world:  What does “dangerous climate change” mean?

It is certain that there is a range of possible consequences resulting from anthropogenic climate change that cannot be quantified and possibly cannot be identified in advance.  The observed time series are too short in order to make statistically significant statements, and the scientific understanding is incomplete. Although this situation probably cannot be fundamentally changed in the near future, there are certain risks based on physical mechanisms that can be documented today. Regarding the uncertainties, it is important not to confuse the risks with projections or even prognoses. 

As an example, an increase of extreme events like heat waves, droughts, floods and extreme cold periods is expected. The reason for this is: 1) an increased amount of energy in a warmer climate system; 2) the speed of anthropogenic climate change; 3) the fact that a warmer atmosphere can absorb more water steam.  Additionally, the rise of the tropic sea surface temperature can lead to stronger hurricane activity, consistent with the observed record years of 2004 and 2005.

A range of risks can be classified as so-called “tipping points” or tipping processes of the climate system. These are processes that react in a very sensitive way to small changes and which have a threshold value for what constitutes an acceptable external disorder.  If this value is exceeded, the dynamics of the system itself possibly leads to irreversible changes.

The melting of the Greenland ice sheet is one possible tipping point.  The ice sheet is fed by snowfall at a high level of up to 3 500 m high ice.  Due to slow outflow of the ice in the lower and therefore warmer heights where it melts, the sheet loses its mass. An imbalance with stronger melting compared to snow accumulation can reduce the height of the entire ice sheet and enlarge the melting place – a self-reinforcing and therefore possibly unstable process.  Satellite records have shown strong fluctuations in the extension of melting places from year to year since 1979.  However, on average, we have observed a rise of approximately 16 %.  Many processes regarding ice sheet dynamics have been insufficiently understood and are not considered in models, which means that great uncertainty remains.  It is clear that total melting of the Greenland ice sheet would increase the sea level by 7 m globally.  The West Antarctic ice sheet would add a further 6 m, the East Antarctic ice sheet would add some more 50 m.

Another possibly unstable process is dense ocean circulation in the Northern Atlantic. The warming or the stronger fresh water inflow in the Northern Atlantic could decrease the water density so strongly that the decline, i.e., the deepwater formation, would be paralyzed.  In such a case – this is a possibility shown in a computer simulation – the climate on the Earth would change rapidly and drastically.  Northern Europe would experience dramatic cooling while the south would become warmer. The sea level would rise by one meter in the Northern Atlantic in addition to the already-observed rise due to the expansion of the water that is becoming warmer and to the water inflow due to the glacier melting.  The impacts for the Atlantic ecosystem and the carbon dioxide absorption of the ocean would be dramatic.

As a theoretical possibility for the Indian monsoon, there is a dynamically very similar mechanism to the one responsible for the possible instability of the ocean circulation.  It could be possible that not only is the current situation with the existing monsoon stable, but that another situation without monsoon rain is persistent too.  A conceptual model shows that a transition between these situations might come about due to high air pollution.

In contrast with future projections, tipping points must be observed within risk assessments.  The probability of destabilization of these tipping processes is often very low, while the consequences are dramatic.  It is a social task, not just a scientific one, to consider carefully which risks are acceptable and which risks are not.