At the 2015 Paris climate conference, 197 countries agreed to “make efforts” to limit the rise in the earth’s average temperature to 1.5 degrees Celsius above pre-industrial levels.

But as early as 2016, which according to the World Weather Organization WMO was the warmest year since weather records began, the global mean temperature was only 0.2 degrees below this limit. Is it then at all possible to prevent the earth from heating up beyond the Paris temperature limit?

The greenhouse effect is driving temperatures higher

First the bad news: The concentration of the greenhouse gas carbon dioxide (CO₂) in the atmosphere is now around 420 ppm. This means that the researchers count around 420 molecules of CO₂ in every million air molecules. Over the past two decades alone, more than 50 ppm have been added.

And this year, too, almost 40 billion tons of CO₂ will be released into the atmosphere from chimneys, chimneys, exhaust pipes and many other sources, which will mean a further increase of around three ppm. Accordingly, the greenhouse effect is gaining in strength and driving temperatures further up.

Temperature stops rising when fossil fuels are no longer being burned

The good news is that once human civilization stopped burning coal, oil, and gas and releasing CO₂, the temperature rise would stop shortly thereafter. For a long time, the researchers had feared something else: Even after emissions were stopped, the earth’s temperatures would continue to rise for a while.

Because the ongoing greenhouse effect of the large amounts of CO₂ that had already entered the earth’s atmosphere over the past decades would cause further warming by a few tenths of a degree.

However, this fear proved to be unfounded. The climate models that predicted this “temperature lag effect” were a bit too simplistic: they assumed that the level of CO₂ in the atmosphere would remain the same after emissions were stopped.

This resulted in the following development: The greenhouse effect keeps throwing the earth’s energy balance out of balance and it absorbs more solar energy than it radiates back into space from the upper limit of the atmosphere in the form of infrared heat radiation.

More than 90 percent of the resulting surplus of energy, which the earth’s climate system continuously captures, is absorbed by the oceans. However, the higher their temperatures rise as a result, the less heat they absorb – and the more heat remains in the air.

The biosphere absorbs CO2 – and counteracts the greenhouse effect

If this effect alone were at work, the temperatures of the layers of air near the ground would continue to rise for a while after CO₂ emissions had ended. However, another effect counteracts this: the oceans not only absorb heat, but also CO₂. And the biosphere of the earth, its soils and forests, even the lithosphere with its rocks, constantly remove CO₂ from the atmosphere.

Of course, this CO₂ extraction from the earth’s atmosphere would continue even if no further molecule of this gas were released during the combustion of coal, oil or gas. Consequently, after an emission stop, the concentration of carbon dioxide in the atmosphere would not remain the same, but would decrease. And accordingly, the strength of the associated greenhouse effect would also decrease.

After climatologists included this cooling effect in their simulations, the high-performance computers delivered a surprising result.

“One of the most important insights of the past few years”

The climate researcher Friederike Otto from Imperial College London rates it as “one of the most important findings in climatology in recent years”: The further warming of the lower layers of the earth’s air in combination with the oceans and their increasing cooling due to the decrease in the greenhouse gas CO₂ are mutually increasing or less precisely cancel. In other words, ending the burning of fossil fuels will also end the further rise in the Earth’s temperatures.

However, the average temperature of the earth will not decrease after CO₂ emissions are stopped. Rather, it will remain at the level to which it will rise for a long time until no more CO₂ enters the atmosphere. So the earlier “zero emissions” are achieved and the less CO₂ is released by then, the better. Because the global average temperatures of permanently over 1.5 degrees above the pre-industrial level would certainly be a catastrophe for our civilization.


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