Climate and climate change
Measurements have clearly shown the direction in which the climate is developing: the average global temperature has risen by about 1 degree Celsius from the pre-industrial age to 2017. How far it will increase by the end of the 21st century ultimately depends on how quickly and extensively humans can change their climate-damaging behaviour. Scientists from various Max Planck Institutes are working on making climate forecasts even more accurate. They are investigating which measures are required to limit climate change to a tolerable level.
The Intergovernmental Panel on Climate Change (IPCC), which also includes Max Planck scientists, gives a clear message: as stated in its Fifth Assessment Report, it is highly probable that greenhouse gas emissions from human activity – especially CO2 emissions from coal, oil, and natural gas combustion – are causing global warming and that this is already being measured and felt today. Even if the development of the average global temperature is still in line with forecasts, it is not yet clear how severe global warming will be by 2100. A key inconclusive factor is human activity. Can we substantially reduce greenhouse gas emissions in the coming decades? Or will the CO2 content of the atmosphere continue to increase? But even the physical formulae used by climate researchers to calculate their forecasts still contain uncertainties. For example, IPCC predictions for global warming indicate a range of 1.5 to 4.5 degrees Celsius for a doubling of the CO2 content compared with the pre-industrial age, when the atmosphere contained 285 parts per million of CO2. The relatively large interval results from numerous imponderables in the climate system. This is because global warming can result in a feedback effect, which can either further aggravate climate change or mitigate it. The climate researchers of the Max Planck Society hope to reduce the uncertainties in the calculations. They are working on robust climate forecasts for the next one or two decades. They are also looking into how the worst consequences of climate change can be averted. How quickly do we need to reduce CO2 emissions in order to limit global warming to 2 or even 1.5 degrees Celsius as agreed at the Paris Climate Conference in 2015? Is geo-engineering (the targeted release of dust that blocks sunlight) a feasible way of achieving this goal?
In view of the rising CO2 emissions, it is becoming increasingly difficult to limit warming to 2, let alone 1.5, degrees Celsius by the end of the 21st century. As researchers at the Max Planck Institute for Meteorology have calculated, this is still possible. But only if people start emitting considerably less greenhouse gas. However, even if all emissions had been completely eliminated in 2017, the average global temperature would still rise by about 1.1 degrees by 2100. The calculations also show that at the current emission rate, it will only take about 15 to 30 years until the risk of exceeding a global warming of 1.5 degrees reaches fifty percent.
Whether the 1.5 degree target will still be met not only depends on political decisions – each individual must also make their own contribution. A study by researchers at the Max Planck Institute for Meteorology investigating the influence of climate change on Arctic sea ice underlines the fact that the contribution of individual humans really is relevant. The scientists have found a simple correlation for how many square meters of Arctic sea ice disappear as a result of a specific CO2 emission. For example, every passenger in an aircraft from Frankfurt to San Francisco leaves a CO2 footprint that melts five square meters of sea ice.
Perhaps such concrete statements on how our own behaviour affects the climate will help persuade people to act. The relatively abstract scenarios of what could happen at the end of the century if we cannot slow down climate change do not seem to be working. This is confirmed by a study by scientists from the Max Planck Institute for Meteorology and the Max Planck Institute for Evolutionary Biology. According to this study, people are more driven by short-term rewards than the prospect of their behaviour preventing catastrophic climate change in the distant future.
This human behaviour, which is still reflected in political decisions, has motivated Hamburg climate researchers to make the threat of global warming more tangible for people today. They are working on models that provide reliable predictions for the coming decades. Such medium-term forecasts are hindered by the fact that the climate is generally subject to relatively large statistical fluctuations. It is therefore difficult to reliably identify clear trends in the near future.
Forecasts of the regional impact of climate change can also show us the urgent need to emit significantly less greenhouse gas. Scientists have calculated how the water balance in Germany will change as the temperature rises. According to their predictions, it will rain more in winter, and there will be more droughts in summer.
Scientists are trying to make plants more resistant to drought in order to reduce the risk of poor harvests in a future with drier summers.
Breeds that can cope better with water shortages are likely to be in demand in southern Europe, where less precipitation is expected as a result of climate change.
In addition to such small-scale adaptations to climate change, researchers are also discussing far more far-reaching and comprehensive measures to respond to global warming: geo-engineering. The purpose of these measures is to cool the earth despite an increasing CO2 content in the air, or to actively remove CO2 from the atmosphere. Such activities are controversial, mainly because although they may slow global warming, they could also have undesirable side-effects.
Geo-engineering raises issues not only relating to climate science, but also to international law. If, for example, a state were to single-handedly release sulphate particles into the atmosphere to screen out sunlight, this would have a global impact and could lead to conflicts with other states in which the measure would have harmful consequences.
Reforestation is being discussed as a possible means of combating global warming caused by the greenhouse gas CO2 and is already being practised in some cases. It is intended to actively remove CO2 from the atmosphere by permanently building up biomass in forests. Scientists at the Max Planck Institute for Meteorology have investigated how much this could limit global warming and whether other proposals for geo-engineering might be more effective. Reforestation is a modification of land use that could potentially slow climate change. However, such alterations often accelerate global warming, such as when tropical rainforests have to give way to pastures. The connections are not so clear all over the world, however. Although forests bind CO2, they have a lower albedo (i.e. they do not reflect sunlight from the Earth’s surface as well as a grass landscape, for example).
What is certain is that the Amazon rainforest is particularly important for the climate system because it stores massive amounts of carbon, which are released as CO2 during clearing and continue to fuel global warming.
The importance of tropical rainforests in the Earth system and the various effects of land use illustrate how complex the interrelationships in the Earth system are. The same applies to the interaction between species diversity and climate.
This complexity in the Earth and climate system makes it difficult for climate researchers to make accurate forecasts. Global warming can trigger developments that it could intensify in the form of feedback. However, some consequences may possibly also mitigate them. Against this background, it is particularly uncertain how the cloud cover of Earth and precipitation will develop if it becomes increasingly warmer on Earth – and also how this change will affect the climate. Max Planck researchers are working to clarify these relationships.
Another unknown factor in the forecasts for the future climate is the feedback between global warming and a possible thawing of the permafrost soils in Siberia. These soils store large amounts of carbon. If the deeper soil layers do not remain frozen in summer, this carbon could escape in the form of greenhouse gas and exacerbate climate change.
Such feedback has always played a major role in climatic changes that have occurred repeatedly throughout Earth’s history. According to all that researchers currently know, the climatic changes of the past have never been as rapid as the global warming we are currently experiencing. Moreover, they have never been triggered by an increase in the CO2 concentration in the atmosphere – and certainly not by an increase caused by humans. However, by studying past developments, climate researchers are learning a lot about the feedback we can expect today.
Extreme weather events – especially droughts – could also lead to a feedback that intensifies the current climate change. This is because plants absorb less CO2 during periods of extreme drought. In general, climate change affects the carbon cycle between the atmosphere and vegetation. Geoscientists refer to this as the biosphere. Scientists at the Max Planck Institute for Biogeochemistry are investigating how this affects the CO2 content of the atmosphere.
It is not yet clear whether such weather extremes have become more frequent and more violent in recent decades. With regard to flooding, researchers at the Max Planck Institute for the Physics of Complex Systems have analysed this question using time series. According to current data, they have not yet observed any trend towards more frequent and severe flooding of the River Elbe, even though exceptionally high water levels were reached in 2002 and 2013.
It has also not been clearly proven that the extremely dry summers of recent years in southern and western Europe, including some parts of Germany, can be attributed to climate change. However, according to climate forecasts, increased and longer-lasting droughts can be expected in certain parts of Europe. Climate change will probably lead to another type of extreme in North Africa and the Middle East. This is likely to result in intolerable living conditions, especially in the affected regions. This will increasingly force people to migrate, as calculations by scientists at the Max Planck Institute for Chemistry in Mainz have shown. One example of the political consequences that climate change can have at national and international level is the civil war in Syria, which can be partly attributed to a drought lasting several years, and which resulted in hundreds of thousands of people fleeing the region.
Climate change might not lead to more hostile conditions everywhere and in every respect. For example, there could be more rain in the Sahara. In some parts of the desert, vegetation is likely to spread.
Times in which current desert areas were more humid and thus greener and less hostile to life already existed in the course of the earth’s history. Such phases may have even played an important role in the history of humankind.
Examples such as these show once again just how much humanity is dependent on the development of the climate – for better or for worse.