Global warming can only be delayed

Climate change is flowing

The flood of the century in Germany in the summer of 2002 "washed" the climate problem into the public eye. Dr. Mojib Latif, until recently a scientist at the Max Planck Institute for Meteorology in Hamburg, warns not to experiment with our planet. Rather, humanity should finally set the course for a sustainable climate policy.

Through their diverse activities, humans intervene in the trace gas balance of the atmosphere and thus influence the global radiation balance. This has long-term consequences for the climate, as the lower air layers and the earth's surface warm up - which leads to the “anthropogenic” human-induced greenhouse effect. Carbon dioxide (CO2). This trace gas is released primarily through the use of fossil fuels such as crude oil, coal or natural gas, and therefore its emissions are closely linked to global energy consumption. Other important trace gases are methane (CH4), Nitrous oxide (N.2O) as well as the fluorine-chlorine-hydrocarbons (CFCs). Carbon dioxide accounts for around half of the anthropogenic greenhouse effect - and the fact that its typical lifetime in the atmosphere is around a century explains the long-term nature of the climate problem. As measurements show, the CO2-The level of the atmosphere has risen rapidly since the beginning of the industrial revolution. Around 1800 it was still around 280 ppm - i.e. 280 CO2-Molecules under a million air particles - this is how we measure 370 ppm today.

Flooded street

It is undisputed that humans are responsible for this increase. A look into the past shows that the CO2-Content is higher today than it has been in about 400,000 years. It was subject to natural fluctuations. But ice cores, i.e. air bubbles trapped in the ice, show that the CO2-Concentration always remained between 200 and 300 ppm. During the last ice age about 20,000 years ago it was around 200 ppm, while during the last warm period 120,000 years ago it was 300 ppm. So we are currently well above the natural fluctuation range.

The earth - a freezer

If the earth had no atmosphere, its surface temperature would be determined solely by the balance between the radiated solar energy and the infrared thermal radiation emitted from the ground and would be around minus 18 degrees Celsius on a global average. Even an envelope of pure oxygen and nitrogen - the two main constituents of the atmosphere - wouldn't change that much. However, certain trace gases, including carbon dioxide, absorb the thermal radiation emanating from the earth's surface and emit long-wave radiation back towards the earth. This leads to additional warming; therefore the global mean temperature of the earth's surface is around plus 15 degrees Celsius: it is thanks to this natural “greenhouse effect” that there is life at all on our planet.

The concentration of long-lived greenhouse gases is increasing systematically: since the beginning of industrialization until today, it has been around 30 percent for carbon dioxide, 120 percent for methane and around 10 percent for nitrous oxide. This drives up the temperature of the lower atmosphere and the earth's surface in the long term. This warming increases with concentration, but is also strongly determined by the reaction of the water cycle - which has both a reinforcing and a dampening effect, as many of its branches depend on the temperature. An increased greenhouse effect therefore leads to changes in precipitation, cloud cover, sea ice extent, snow cover, sea level and extreme weather - and changes the global climate. In the process, humanity, and the Elbe flood made this clear, above all felt the change in extreme values.

Delayed reaction

The strong increase in greenhouse gases - especially carbon dioxide - in the atmosphere intensifies the greenhouse effect and means global warming on the earth's surface. This begs the question of what climatic changes can already be observed today. It applies here that the climate responds to external impulses with a delay of several decades. So we shouldn't assume that we are already observing all of the expected warming today. But reconstructions of the temperature in the northern hemisphere over the past 1000 years reveal a clear warming trend over the past 100 years. However, the temperatures up to 1900 were mainly derived from indirect methods such as the analysis of growth rings on trees, which leads to considerable inaccuracies. But even then, assuming the maximum uncertainty, the decade from 1990 to 1999 remains the warmest of the past 1,000 years.

Further statistical and model-based indicators show that the observed rise in temperature over the past decades is very likely due to humans. In the past, there were repeated climatic fluctuations that could not be ascribed to humans - such as the medieval warm period and the little ice age. However, compared to the rise in temperature in recent decades, these deviations were, at least on a global scale, significantly weaker.

The role of the sun in global warming is repeatedly asked. In fact, solar radiation fluctuates - among other things with sunspot activity. The number of spots increases, so does the radiation, and at the same time the solar spectrum shifts into the short-wave, ultraviolet range. Two cycles are observed: First, the so-called Schwalbe cycle with a period of eleven years and a measured amplitude of about 0.1 percent; on the other hand, the Gleissberg cycle with a period of around 80 years and an estimated amplitude of 0.2 to 0.3 percent of the total irradiation. The solar radiation can fluctuate up to 0.6 watts per square meter. For comparison: the additional greenhouse effect due to the increased concentration of carbon dioxide, methane, fluorine-chlorine hydrocarbons and nitrous oxide is currently around 2.4 watts per square meter.

Averaged over the last 100 years, the solar constant has increased: The researchers estimate it today to be 0.25 percent higher than 100 years ago. Model simulations show that only 0.2 degrees Celsius - and thus a third - of the global warming observed since 1900 is due to the sun. So the solar variability alone cannot explain the observed temperature increase of around 0.7 degrees Celsius: the majority of global warming is caused by humans. This is consensus in international climate research and is documented in the report of the Intergovernmental Panel on Climate Change (IPCC, see info box below). In view of this fact, it can no longer be a question of whether humans influence the climate, but only to what extent climate change can be contained.

Computer simulations allow predictions about the development of the global climate. These models quantitatively describe the physical interactions between the atmosphere, ocean, sea ice and land surfaces. Such models require, among other things, the concentrations of the most important long-lived greenhouse gases as parameters, while the concentration of short-lived aerosols - which is closely related to the formation of clouds and precipitation - is calculated within the simulation.

A model developed at the Max Planck Institute for Meteorology in Hamburg was used to simulate the climate from 1860 to the end of the 21st century. For the period from 1860 to the present day, the observed concentrations or emissions of the relevant trace gases were prescribed; for the future it was assumed that today's trends will continue unabated. This simulation yielded global warming of around 0.7 degrees Celsius from the late 19th century to the present, which agrees well with the observations. Global warming up to the middle of the 21st century - the difference between the decade means from 2040 to 2049 and from 1990 to 1999 - is around 0.9 degrees Celsius. The continents are warming by 1.4 degrees Celsius, which is about twice as much as the oceans. By the year 2100, global warming is up to 3 degrees Celsius, depending on the scenario. Together with the global warming of around 0.7 degrees Celsius, which is already measurable today, this would almost correspond to the temperature difference from the last Ice Age to today. This would mean a rapid global climate change for which there would be no analogue in the last million years.

But even in a scenario in which greenhouse gas emissions are reduced to a fraction of today's emissions by the end of this century, the global mean temperature of the earth will still rise by just under a degree. This is due on the one hand to the inertia of the climate system - especially the oceans - but on the other hand because there are already large amounts of greenhouse gases in the atmosphere, the levels of which only decrease slowly over many decades.

Global warming leads to increased atmospheric water vapor as well as increased water vapor transport from the oceans to the continents - and thus to increased precipitation there. However, these changes vary greatly from region to region. Generally more precipitation is recorded in high latitudes and in parts of the tropics, while the less rainy subtropics continue to dry up. This worsens the discrepancies between the humid and dry climates on earth. This also applies to Europe, albeit with seasonal differences: While summer precipitation is decreasing almost everywhere in Europe, a pronounced north-south gradient is predicted in winter with a decrease in southern Europe with little precipitation and an increase in central and northern Europe with high precipitation.

This increase is accompanied by intensified winter storm activity over the north-east Atlantic and stronger westerly winds that bring moist air from the Atlantic. What is noticeable is an accumulation of heavy precipitation both in winter and in summer and thus an increased probability of flooding. This even applies in part to the Mediterranean region, where mean rainfall is decreasing. The cause is probably the higher water vapor content of the atmosphere as a result of the warming, which leads to higher amounts of precipitation in extreme weather conditions. Therefore, extreme floods are to be expected more frequently in the future.

Coarse mesh models

The results depend crucially on the chosen scenario, i.e. on the assumptions about the future development of the world population, industrialization and the consumption of fossil fuels. In addition, models only ever provide an approximate simulation of the complex real climate system. In general, the informative value of the models becomes weaker the smaller the area under consideration is. For example, regional details within Germany can be recorded less precisely than differences between northern and southern Europe. This is mainly due to the still relatively coarse “mesh size” of the global climate models of a few hundred kilometers; it does not allow mountains - like the Alps - to be resolved well or small-scale processes - such as the formation of clouds and precipitation - to be adequately represented. In addition, the models are currently still incomplete. This means that possible changes in both the vegetation and the mass of the inland ice are not taken into account.

As a result of the climate change outlined, the vegetation could change, and this in turn would have an effect on the temperature of the land surface. Such vegetation-dynamic feedback will probably be taken into account in the next generation of climate models, as will the interaction with chemical processes in the atmosphere. It should be noted, however, that the simulations reproduce the large-scale and long-term changes in the climate relatively reliably, despite all the uncertainties described. This is proven by simulations of past climatic conditions.

The climate issue is now also at the top of the global political agenda. On December 10, 1997, 159 signatory states to the United Nations Framework Convention on Climate Change unanimously adopted the so-called Kyoto Protocol. As required in the Berlin mandate at the first Conference of the Parties in April 1995, the first implementing provision for the Climate Convention was at least formulated. It forces the industrialized countries - currently 39 - to reduce their greenhouse gas emissions by an average of 5.2 percent by the period 2008/2012 (based on the emissions in 1990) if the parliaments ratify the protocol or the governments accede to the protocol.

Act and negotiate

The Kyoto Protocol becomes binding under international law if at least 55 of the currently 175 contracting states, which at least 55 percent of all CO2-Emissions from 1990 have been ratified. In addition to carbon dioxide, emissions of other greenhouse gases can also be reduced by converting them into CO2-Equivalents are taken into account.

The European Union must reduce emissions by an average of 8 percent, more than the USA with 7 or Japan with 6 percent. However, it is questionable whether the US will ratify the protocol. Russia should only stabilize, and Norway may even grow. These different reduction rates are the result of demonstrably different conditions, but also partly a result of the negotiating skills of individual countries.

With the Kyoto Protocol, mankind begins with a kind of earth system management. The procedural rules have yet to be clearly defined. For example, it must be clear what percentage of the CO2-Reduction a country is allowed to settle with the purchase of emission rights from another country, which has reduced beyond its obligation. Furthermore, it must be clarified how much carbon dioxide is bound by a newly reforested forest area and thus can be deducted from the emissions through the use of fossil fuels. The Kyoto Protocol in its current form by no means provides the climate protection that climate researchers believe is necessary. In order to prevent serious climate changes in the next 100 years, the emission of greenhouse gases would have to be reduced to a fraction of today's value by 2100. The annual conferences of the contracting states, however, offer the opportunity for improvements - this was also the case with the Montreal Protocol, the implementing provisions of the Vienna Convention for the Protection of the Ozone Layer.

In the future, however, more weight will have to be attached to the introduction of renewable energies, because only this (especially solar energy) is available in unlimited quantities. It is a mistake to believe that the necessary reductions in greenhouse gas emissions can only be achieved by increasing efficiency. It is important to convert the energy industry in the long term in the direction of renewable energies - also because fossil fuels are limited and mankind's energy needs will increase.

Since the climate only ever reacts to long-term strategies, the transformation of the economy can take place gradually over the next 100 years. It would be important to exhaust all potential for saving energy today and to take the path to lower greenhouse emissions. In this respect, the Kyoto Protocol marks a first and very important step in the right direction.

Setting the course for sustainable development today also makes economic sense: it is generally cheaper to take precautionary measures than to settle more and more climate-related damage in the future. The Elbe flood made this clear. Furthermore, we should not experiment with our planet as a matter of principle. The past shows that there are many surprises in store. For example, the ozone hole over the Antarctic was not predicted by any scientist, although the ozone-depleting effects of the chlorofluorocarbons were known. The climate as a non-linear system can show unexpected reactions in the event of strong deflections: It would be unwise - and irresponsible - to take it down on it.