IPCC publishes new report on the Physical Science basis of Climate Change

Dr Mike Morecroft

You can’t fail to have noticed that the Intergovernmental Panel on Climate Change published a major report last week – websites, radio, TV and newspapers were full of it. Of course climate change reports are by no means rare these days, but this one is a big deal. Every 7-8 years the IPCC produces an ‘Assessment Report’ which is a comprehensive overview of climate change, its impacts, adaptation and mitigation. This is the 6th such Assessment Report. The Assessment is in three parts. The present report is just the first part and only covers the physical science of climate change itself. The 2nd one is on impacts and adaptation (I’m one of the ‘Coordinating Lead Authors for it’) and will come out next spring. A 3rd report covers mitigation (emissions reductions) and will also be published next year.

So what did we find out in this new report? It is impossible to distil nearly 4000 pages into a few sentences, but these are some of the conclusions I take away.

  1. The evidence of anthropogenic climate change is stronger than ever – ‘unequivocal’ is the word used to describe it in the Summary for Policy Makers, which encapsulates the key points. IPCC reports use words like this very carefully and we’ve come along way from the equivalent statement in 1995 that ‘the balance of evidence suggests a discernible human influence on global climate’! Warming of approximately 1oC has already occurred since preindustrial times.
  2. Our understanding of past climates has improved and allows us to see present warming in a clear long-term perspective. The report concludes that ‘the scale of recent changes across the climate system are unprecedented over many centuries to many thousands of years’.
  3. The evidence on observed changes in climatic extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and their attribution to human influence, has strengthened substantially in recent years. As someone who has worked a lot on the impacts of climate change on natural systems, this is one of the most important points to me: the ecological impacts of extreme events are often larger than those of gradual changes in temperature.
  4. Keeping global warming to 1.5 °C or 2°C is possible but will require immediate, rapid and large-scale reductions in greenhouse gas emissions. This is a critical issue for the UN Framework Convention on Climate Change as we head towards the crucial COP26 meeting in Glasgow in November. Difficult decisions will need to be taken if we are to avoid the worst effects of climate change in the decades to come.
  5. As emissions increase, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere. Up to now rises in CO2 in the atmosphere have been lowered significantly by plants taking it up and the sea absorbing it. There is already evidence of this slowing in some places and as the world warms we may see the rise in CO2
  6. Sea level rise relative to 1995-2014, is likely to be in the range 0.28-0.55 m by 2100 under lowest emissions scenario (consistent with approximately 1.5°C temperature rise) and 0.98-1.88 m under the highest scenario. Even the lower end of the scale there will be impacts.
  7. Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level Over the next 2000 years, global mean sea level will rise by about 2 to 3 m even if warming is limited to 1.5°C. This will lead to substantial change in coastlines. At 5°C of warming it would be expected to be in the range 19 to 22 m!

At one level there is nothing particularly surprising in this report – it confirms and adds detail and confidence to earlier reports. It does however clearly set out how big a challenge it will be to meet the Paris Agreement commitment of keeping global mean temperature rise to well below 2°C and pursuing efforts to hold it to 1.5°C. However, it also reinforces the importance of doing so. There is now a lot more confidence in understanding the implications of climate change not just for temperature rise, but also extreme events such as droughts, floods and wildfires, loss of ice and rises in sea level. It will be a powerful influence on COP26 and reinforces the importance the Net Zero agenda.

So what do we do about this growing threat? The IPCC will come onto that in the next two reports but it’s a question we all need to ask ourselves.

Footnote. JRI members will be pleased to know that this report is dedicated to our former President and founding chairman, Sir John Houghton, who was one of the leaders of the IPCC through its early years. Perhaps the most fitting of his many accolades!

Dr Mike Morecroft is JRI President


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