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Climate change as a planetary boundary
Heat waves, floods, and droughts: Climate change is advancing globally, and its effects are also becoming increasingly noticeable in Germany. The risk of abrupt and irreversible changes to the Earth system rises with every tenth of a degree Celsius of additional warming. Even an increase of less than two degrees Celsius means vastly greater risks for society and the environment. We have already exceeded this planetary boundary.
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Julia Blenn / Helmholtz-Klima-Initiative
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Julia Blenn / Helmholtz-Klima-Initiative
At the UN Framework Convention on Climate Change about 30 years ago, almost all of the world’s countries agreed to stop climate change and avoid the risk of dangerous changes to the Earth system. Rising temperatures could destroy the livelihoods of hundreds of millions of people. People living in coastal regions, in areas already suffering from drought, as well in cities, would be particularly affected.
In the planetary boundaries concept, climate change is one of the three most important factors affecting our future, along with the loss of biodiversity and the spread of “novel entities” (substances such as chemicals and plastics) in the environment.* Recent findings announced by the IPCC in 2022 indicate that the planetary boundary for climate change has already been exceeded and that humanity urgently needs to take action to return to safe conditions.
Our emissions from energy generation, transport, agriculture and other activities have caused the concentration of carbon dioxide in the atmosphere to rise well above pre-industrial levels.
Extreme weather events like the heat wave in India in the spring of 2022 with temperatures around 50°C, the forest fires worldwide, or the flood disaster in Germany in 2021, are already showing how we humans are pushing ourselves to our breaking point – even with a global average temperature increase of a mere 1.2 degrees Celsius. But as we are seeing, even such a seemingly small temperature increase can have severe consequences, since the temperature doesn’t rise uniformly over areas or time.
If we further exceed the planetary boundary for climate, we could trigger abrupt changes in the climate system. Even a temperature increase of 1.5 to 2 degrees Celsius, which we have nearly reached, is likely to trigger feedback processes in the Earth system that will exacerbate climate change in ways we can no longer control; we would reach what are called tipping points for the climate system.
The greenhouse effect: driving climate change
The force driving climate change is the increase in greenhouse gases, i.e. the rising concentration of carbon dioxide and other gases like methane in the atmosphere. They trap heat in the atmosphere, changing Earth’s energy balance. Scientists call this radiative forcing. This means that less of the heat caused by solar radiation on Earth can escape through the atmosphere.
There has always been a greenhouse effect, and it helps keep the temperature on Earth at a level that makes life possible. As with the glass roof of a greenhouse, sunlight enters the atmosphere but not all of the heat can escape.
But if we increase the greenhouse effect by emitting greenhouse gases like carbon dioxide and methane into the atmosphere, temperatures get warmer. What makes tomatoes and other vegetables grow better locally can have disastrous consequences at the global level because greenhouse gases increase the greenhouse effect and can heat up the atmosphere considerably.
How can we return to the safe zone?
To adhere to this planetary boundary, we need to reduce our emissions of carbon dioxide to net zero. This means giving up fossil fuels and switching to other energy sources like wind power, photovoltaics, hydropower, or geothermal energy. Where avoiding emissions is impossible, we need to remove them from the atmosphere (negative emissions). Our goal should be to develop a carbon-neutral society as soon as possible in order to return to a safe operating space for human civilization in the long run.
Venus: the runaway greenhouse
The atmosphere of our neighbor planet, Venus, is comprised of more than 96 percent carbon dioxide. That is so much carbon dioxide that even Venus’s thick clouds, which reflect 95 percent of the incident sunlight, are unable to keep the planet from reaching temperatures as high as 465 degrees Celsius. Here on Earth, carbon dioxide is a trace gas making up less than 0.05 percent of our air – about 400 parts per million molecules.
According to the European Space Agency (ESA), we don’t fully understand what caused this runaway greenhouse effect on Venus. What is clear is that the large amount of carbon dioxide it its atmosphere led to these high temperatures. Although, fortunately, we need not expect such a hellish scenario here on Earth, Venus provides an impressive demonstration of the potential impact an extreme greenhouse effect.
Scientific editing: Tido Semmler, Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research.
*Interview with Johan Rockström on Living on Earth (2016).
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