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Global temperature drops may lag behind carbon reductions

While pressure mounts to quickly reduce the amount of carbon produced by human activities, researchers warn that it may take decades for the reductions in global temperature to be measurable.

A new study suggests that if we only use the Earth’s surface temperature to measure whether emissions cuts lead to a slow-down in global warming, we may need to wait for decades before we can establish with certainty the effect on temperature of the reduced emissions.

This is partly because the Earth’s climate responds slowly to changes in emissions, and partly because of natural variability in annual mean temperatures.

“Human-induced climate change can be compared with a tank ship at high speed and in big waves,” said researcher Bjørn H. Samset from the CICERO Center for International Climate Research. “If you want the ship to slow down, you will put the engine in reverse, but it will take some time before you start noticing that the ship is moving more slowly. It will also rock back and forth because of the waves.”

Over the past 50 years, the Earth’s surface temperature has on average increased by 0.2°C every decade, mainly due to human induced emissions of greenhouse gases. However, from one year to the next, there are also large variations which on a similar scale.

“If we are to reach the Paris Agreement ambition of limiting global warming to no more than 2°C – or less – the first step will be to slow down the warming process,” Samset said. “But although the necessary emissions reductions are effective from day one, it will take some time before we can measure this effect with certainty.”

“Just like it took some time to establish that global warming is happening, we will need to have some patience before we can determine that the actions taken to limit global warming are having the desired effect.”

The researchers believe that even in the most optimistic scenarios, there is a risk that it will not be possible to establish that the emissions cuts have had an impact on global warming before 2035 even though this does not mean that the emissions reductions are not having the desired effect.

“The effects of climate cuts can be compared with those resulting from social distancing during an epidemic. They work from day one, but because of the incubation time, it will take some time before you can see the effect on the infection rates,” he said. “Similarly, all reductions in warming emissions will lead to less heat being absorbed, but it will take some time before we can measure this effect.”

“Luckily, there are other methods we can use instead. We can estimate emissions reductions very quickly, and if there is a slower increase in the amount of greenhouse gases in the atmosphere, we will see it.”

“But for temperature, which is what we really care about – and which, among other things, has an impact on the increased frequency and severity of extreme weather – it will take decades before we will be able to measure the effect.”

The greenhouse gas reductions necessary to mitigate climate change in line with the goals of the Paris Agreement will require rapid, major changes in society, businesses, and the energy sector. The researchers warn that although this must happen quickly, we must have realistic expectations about how long it will take before we see a temperature effect of these changes.

Last month the International Energy Agency proposed a $3tn green recovery plan that countries could implement to revive their economies while sticking to carbon pledges.  

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