|   Nature


  |   Nature


How to Think About 1.5 Degrees

point degrees in Paris

Astonishment was universal last December when the Paris Agreement on climate change included the aspiration to limit warming to 1.5 degrees above pre-industrial levels, a much tougher target than the standard of 2 degrees, now seen as too risky.

It was a remarkable triumph for a long campaign by the small island states, proving that even tiny nations, armed with a powerful moral case, can change the world.

But what does a global aim of 1.5 degrees mean? Is it achievable? How much difference would it make? A conference at the University of Oxford two weeks ago brought together leading scientists to begin to answer these questions.

No one can give firm answers, but some surprising observations emerged at the conference. One thing is clear: given the vast quantity of carbon dioxide already in the atmosphere, with more still to come, limiting warming to 1.5 degrees will require ‘negative emissions’.

Negative emissions technologies aim to draw carbon dioxide out of the atmosphere and store it safely. Some proposed technologies include machines that extract carbon dioxide from the air, concentrate it and then somehow (the answers are vague) store it. At a scale to make a significant difference, a huge infrastructure of carbon-sucking machines, concentrating equipment and pipelines would need to be built.

The most commonly mentioned method of negative emissions entails generating electricity by burning biomass – mainly crop waste, wood waste, and crops grown for the purpose – capturing the carbon dioxide from the emissions and storing them underground.

It’s estimated that to make a substantial difference to global warming huge expanses of land would have to be given over to growing biomass crops. This risks depriving poor people of food crops and destroying ecosystems as swathes of land are converted to growing biomass for energy.

So here is the first troubling prospect. Although warming of only 1.5 degrees would result in much less harm to the climate than 2 degrees, it’s possible that the ecological damage caused by the negative emissions projects needed to get there may exceed the benefits, at least for some. The ecosystem costs of the emission reduction pathways may outweigh the benefits of lower warming.

So while the overall goal of climate negotiations is to avoid ‘dangerous climate change’, perhaps it needs to be changed so that the goal becomes to ‘minimize dangerous change to the Earth System as a whole’, a dramatic shift in how we think about the issue.

It must begin soon

For a 1.5-degree goal, large-scale negative emissions activity would need to begin soon, before 2030, and expand rapidly, so that by 2050 or sooner the amount of carbon sucked out of the atmosphere would have to exceed the amount emitted into it from fossil fuel burning.

No one is confident it can be done. Some suggest that when the Intergovernmental Panel on Climate Change has included negative emissions in its future emissions scenarios it is not much more than a ‘fudge factor’ to make the 2 degrees limit seem possible.

Apart from the cost, the biggest obstacle to negative emissions technologies is what to do with the captured carbon. Although it’s fairly easy to extract carbon dioxide from the air, no one has yet come up with a feasible and economic way of storing billions of tonnes of it. It must be done safely and it must stay there for thousands of years, without leaking out.

Some years ago governments became excited at the idea of pumping it into geological formations, but pilot projects around the world have been abandoned because they ran into technical problems and cost blowouts. Now it’s thought that storing carbon dioxide underground on a large scale is decades away.


The world has already warmed by 1 degree and momentum in the climate system will almost certainly see the world reach 1.5 degrees, perhaps as early as 2030. So if our goal is to limit warming to 1.5 degrees there will be an ‘overshoot’, taking warming to at least 2 and perhaps 3 degrees, before the average global temperature can be brought back down.

Here is the second troubling possibility. If the world warms by 2 or more degrees will feedback effects kick in – such as unstoppable melting of the Siberian permafrost, which could send more greenhouse gases into the atmosphere, making it virtually impossible to stabilize warming at 2 degrees, let alone 1.5.

And if we could overshoot and return to 1.5 degrees, would ecosystems and vulnerable plant and animal species be able to survive the period of perhaps three or four or five decades of overshoot. Scientists hope that ecosystems possess ‘temporary resilience’ during the overshoot period so that they can bounce back when cooler conditions return.

Equally troubling, for those creatures and ecosystems that do manage to adapt to an environment 2 or 3 degrees warmer, could they cope in a cooling environment as the global temperature is wound back to 1.5 degrees?

When the nations of the world in Paris adopted the 1.5 degree aspiration the politicians were well ahead of the scientists. Now the scientists are scrambling to catch up.

This article was first published by Scientific American.

The ConversationThis article was originally published on The Conversation. Read the original article.

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