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Every seven or so years the Intergovernmental Panel on Climate Change (IPCC) undertakes a comprehensive synthesis and assessment of the scientific literature to produce a series of reports on climate science. The first of these, on the physical science basis, was published this week. Two subsequent reports on impacts and adaptation, and on mitigation will follow in early 2022.
This massive endeavour permits a holistic and comprehensive snapshot of the state of our collective scientific knowledge in a manner that is perhaps unique in any field of science. Robustness is assured through multiple rounds of review and the careful selection of broad author teams. The summary for policy-makers is then signed off word by word by government delegations who can suggest wording changes. However, final acceptance rests with the authors.
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It is the holistic nature and robust processes that give the report its strength: this is so much more than a single new scientific paper.
But what are the most critical points? As an author of the report, here are my take-homes of what matters in a national context.
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The most recent decade globally was about 1.1 degree warmer than the latter half of the 19th century, and humans are assessed as being responsible for all of that historical warming.
Changes in a whole host of aspects of the climate system, including multiple aspects of the atmosphere, ocean, frozen planet and living planet, point unequivocally to rapid changes in the climate system. Recourse to evidence from ice cores, tree rings, etc, highlight how both the rate of change and the current state of the climate system are unprecedented in at least thousands of years and possibly very much longer.
No corner of the world will be spared from increasing impacts of changes in such extremes unless and until we halt further warming
We truly are heading into climate territory uncharted in human history.
It is for the first time concluded by the IPCC that human influence upon the climate system is unambiguous.
This also now goes beyond the long-term changes to finding human influence in many of the extreme heatwaves, extreme rainfall events and droughts that we are increasingly seeing around the world.
And the report is clear that these changes will become progressively more pronounced unless and until we stop the emissions of greenhouse gases into the atmosphere.
Extremes
No corner of the world has been spared the impacts of climate change to date. And no corner of the world will be spared from increasing impacts of changes in such extremes unless and until we halt further warming.
The report makes clear that many aspects of the climate system, such as global surface temperature or rainfall, respond quickly. For these aspects we really can stabilise the climate system within our lifetimes. But there are other aspects of the climate system, the oceans and the ice sheets, that respond much more slowly. These will take centuries to many thousands of years to catch up.
We know why the changes we have seen have occurred: emissions of carbon dioxide, methane and other greenhouse gases
Continued sea-level rise will, therefore, not be a challenge just for our children and grandchildren but for very many generations to come on this island. The changes this century are likely manageable, but the changes committed to are potentially multi-metre sea-level rises that will, eventually, force very hard choices on our many coastal cities.
The past provides windows, albeit imperfect, on our potential future here: the last time temperatures were as warm today sea level was probably 5 metres to 10 metres higher; and the last time carbon-dioxide was sustained at present-day levels sea levels were 5 metres to 25 metres higher. And it gets worse for earlier periods of higher temperature/carbon-dioxide.
The choices we make today will reverberate for millennia to come.
We know why the changes we have seen have occurred: emissions of carbon-dioxide, methane and other greenhouse gases; with carbon-dioxide in 2019 almost 50 per cent above 1750 values and methane almost 150 per cent above those levels. These changes are much larger than the repeated natural changes in these gases between glacial and interglacial states over the past 800,000 years.
Reductions
It follows that we know how to limit further changes. Unless there are immediate, rapid and sustained reductions in greenhouse gas emissions, limiting warming to 1.5 degrees will be beyond reach.
Even in the most optimistic of the five scenarios assessed by the report, in which we reach net negative emissions by 2050, we will probably reach and exceed by a small amount 1.5 degree warming before returning under this level by 2100.
Unless global emissions reach net zero by mid-century and become net negative thereafter, we are unlikely to keep global warming below 2 degrees.
To limit global warming strong, rapid and sustained reductions in carbon dioxide, methane and other greenhouse gases are, therefore, necessary.
Methane, by a best estimate, has contributed 0.5 degrees of warming to date. Because methane has a relatively short lifetime in the atmosphere – the matter of a decade or two – methane reductions would have immediate climate benefits.
Methane reductions would also improve air quality and thus have human health co-benefits.
Unless someone like you cares a whole awful lot, nothing is going to get better. It's not
Carbon-dioxide, on the other hand, stays in the atmosphere for a very long time. Warming will, therefore, only cease when carbon-dioxide emissions reach at least net zero and other greenhouse gases are at least stable.
The potential risks associated with low-likelihood, high-impact outcomes are also highlighted. For example, although a collapse in the Atlantic overturning circulation which drives our moderate climate – keeping our winters mild and our summers cool – is unlikely, were it to occur there would be very substantial impacts. While we cannot know whether such outcomes will occur, it is the case that such outcomes become increasingly likely the more the climate system warms.
Climatic events
This also applies to extreme climatic events. Already some events have been assessed to have been impossible without human interference in the climate system, and this will increasingly become the case. Given the very substantial impacts that would be associated with such events and outcomes, the precautionary principle strongly applies.
The current generation is at a pivotal moment. We now know beyond any doubt that our current lifestyle, principally but not exclusively our reliance upon fossil fuels, is altering our climate in profound and increasingly alarming ways, and that it will get worse unless and until we cease net emissions of these gases.
We also know that the impacts will affect not just us, our children and our grandchildren but many generations to come.
As subsequent reports will make clear, we have solutions to hand. I am reminded of the quote from The Lorax: “Unless someone like you cares a whole awful lot, nothing is going to get better. It’s not.”