Global warming will take longer than first thought

26 September 2017

Interview with 

Joeri Rogelj, ETH Zurich

GLOBAL-WARMING

Illustration of rising sea levels

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Limiting the increase in global average temperatures to one and a half degrees Celsius above pre-industrial levels was one stated goal of the 2015 Paris Climate Agreement. Many said it was unachievable. But now a new analysis suggests we may have some breathing space. Michael Wheeler spoke to ETH Zurich climate scientist Joeri Rogelj...

Joeri - In the Paris agreement there is a long term temperature goal aiming to keep warming well below 2 degrees, and we also aim at limiting it to 1.5. So, basically, this is annual global mean temperature increase relative to pre-industrial levels, somewhere mid 19th century.

Now the question, of course, arises how can we this? Literature has shown that the warming we see is nearly linearly proportional to the total amount of carbon dioxide that we emit into the atmosphere. That means there is a certain maximum amount of carbon dioxide that we are allowed to emit into the atmosphere ever, also often referred to as a carbon budget.

Michael - So the carbon budget is key to being able to reach that target. What was it that you did to calculate what that carbon budget might be?

Joeri - To figure out how much carbon we can still emit, we basically use three different modeling approaches. One is a simple climate model; another method is a model of intermediate complexity which was used to verify the simple climate model. And finally, we also used results from the most complex, state of the art earth system models.

And in the past, carbon budgets were often calculated relative to pre-industrial so that means that small errors and discrepancies they accumulate over time. So what we did, we basically reset these uncertainties by expressing the carbon budget relative to today. What we find if we take into account where we are today, we get to a budget of around 700 to 800 billion tons of carbon dioxide. This corresponds to roughly 20 years of climate emissions.

Michael - So previous carbon budgets gave us far less time?

Joeri - Exactly. Previous carbon budgets gave us far less time, and basically suggested that keeping warming to below 1.5 was almost a geophysical impossibility.

Michael - An increase in temperature of 1.5 degrees seems like a small increase in temperature. Why are such small increases in temperature so detrimental to our climate?

Joeri - One has to be aware that these increases are global average increases. In general, people are not necessarily interested in average temperature increase. What we are interested in is the risks that can arise with, for example, extreme events. There are clear shifts in the extremes.

Michael - Yes, okay. As the average temperature shifts, so too does the extreme end of the spectrum shift so that we’re experiencing more extreme weather events; is that correct?

Joeri - Yes, that’s correct. In some regions this is a very clear trend.

Michael - Your analysis has given us a larger carbon budget and some more hope, and I think there’s two ways that that message can be taken up. One one hand if people believe that we’ve gone past a point of no return, then they also believe that efforts towards conservation will be meaningless. Your analysis steps us back from that, which I think is a good thing but, on the other hand, it may have the potential to relax people a bit more about climate change. I’m not sure which one of those will be most prevalent; what do you think?

Joeri - Our research really put the possibility of limiting warming to 1.5 degrees back on the table and I think that’s a hopeful message, but that doesn’t really mean that the pressure is off. It still requires actions way beyond the pledges that are currently on the table by countries and global carbon dioxide emissions need to be reduced immediately, to become zero by mid century, and that’s an incredibly challenging feat.

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