of warming is delivering devastating extreme weather events, dying ecosystems, increased species extinction rates, the unstoppable melting of the Amundsen Sea sector of the West Antarctic Ice Sheet, and drought induced food and water shortages, amongst other outcomes. But the full warming effect of our emissions to date won’t be felt for several decades, or centuries in the case of rising sea levels.
of warming is only 10 years away, analysis of the latest climate science concludes. Further warming is inevitable, even as we reduce emissions because aerosols — a by-product of burning fossil fuels — have a short-term cooling impact, of around a week, estimated to be in the range of ~0.5–0.8°C. For now, these aerosols are lessening the warming impact of increasing levels of greenhouse gases. But reducing the use of fossil fuels — which we must do — will also reduce the production of aerosols, and push up global temperatures as their cooling mask is removed.
is the upper limit of warming in the Paris Agreement pledge. But emissions reductions via the Intentional Nationally Determined Contributions (INDCs), proposed after the Paris Agreement, will not stop warming at 2°C. The present INDCs would not prevent warming of around 3°C, and up to 5°C when likely carbon cycle feedbacks are included. With greenhouse gas emissions now rising to record levels, delivery of even the promised INDCs is not guaranteed.
of warming, among other impacts, would trigger the loss of both polar ice caps, eventually resulting in a 70 metre sea level rise. A 4°C future is, according to Professor Kevin Anderson of the UK Tyndall Centre for Climate Change Research, “incompatible with an organized global community, is likely to be beyond ‘adaptation’, is devastating to the majority of ecosystems, and has a high probability of not being stable”. He goes on, “If you have got a population of nine billion by 2050 and you hit 4°C, 5°C or 6°C, you might have half a billion people surviving”.
We now face an unacceptably high risk of global warming beyond 4°C. Ongoing emissions increase the likelihood that thawing permafrost will trigger the release of Arctic carbon stores. As the Arctic warms so does the permafrost, which covers 20% of the northern hemisphere landmass and has mostly been frozen for half a million years. Thawing organic matter triggers bacterial decomposition that releases large quantities of methane — the most potent greenhouse gas. The methane creates further warming which melts more permafrost, producing more methane. This is the permafrost carbon feedback cycle.
But recent research on thawing permafrost indicates that reducing emissions quickly can possibly delay triggering the permafrost carbon feedback cycle.
The quicker emissions are reduced to zero, the lower the total in the atmosphere, the less likely warming will trigger unstoppable permafrost, the less we have to draw down, and the lower the temperature rise after aerosol cooling is removed.
“Taking a plane is the fastest and cheapest way to fry the planet” — Bill Hemmings, aviation director Transport & Environment NGO campaigning for cleaner transport in Europe
Even reducing emissions much faster than INDC pledges only gives us a two-in-three chance of getting back to the 1.5°C level of warming, according to the United Nations Environment Program in their report The emissions gap 2017. But 1.5°C is far from safe.
The reality is that there is no ‘carbon budget’ — no more carbon we can burn — if we want warming constrained to below the 1.5°C level. Every tonne of emissions we emit from here on in has to be drawn back down out of the atmosphere.
So, emissions must get to zero tonnes as fast as possible, that is, at a speed unconstrained by the ‘business-as-usual’ politics that today characterises climate policy making. And, because our past emissions have already taken warming to 1.1°C, drawdown of those previous emissions is necessary to get warming back to the safe zone — well below 1°C.
But such cooling of the planet is only possible after warming emissions are reduced to zero, so emissions drawdown is not a substitute for emissions reduction.
In 2017 James Hansen, previously NASA’s chief climate scientist, with others, wrote Young people’s burden: requirement of negative CO2 emissions, a paper researching drawdown, or negative emissions. They found that to achieve 350 parts per million of CO2 in the atmosphere by 2100 — the upper limit to the safe zone — at least 150 gigatonnes of previous emissions already in the atmosphere need to be drawn down, at the same time as emissions are reduced to zero by 6% each year.
Humanity faces an existential crisis not at some time in the future, but right now. We must respond immediately.