Heatwaves, 'Hothouse Earth' and putting the heat on governments


Man dressed in an office outfit with sweaty shirt walks in sunshine

Q&A with Joanna Haigh, Professor of Atmospheric Physics and Co-Director of the Grantham Institute – Climate Change and the Environment.

She reflects on 2018's weather highs and lows and asks when it's appropriate to blame climate change, and if so, what can be done about it.

The heatwave that we experienced in the United Kingdom over recent weeks was part of a pattern of unusually hot and dry weather seen over many parts of the globe. Associated with it were widespread wildfires, with loss of lives, homes and livelihoods. The two questions that many people have been asking are 'What has caused the heatwave?' and 'Is it due to climate change?'

When is a heatwave a heatwave?

There is no precise definition of a heatwave but it is generally taken to be a period of a few days during which the daytime maximum and night-time minimum temperatures are unusually high for a particular location. Quite often, as in the recent situation, heatwaves occur because the jetstream (a band of strong winds blowing eastwards high in the atmosphere) weakens and becomes more wavy, or even splits into two. This can result in the weather systems that it normally drives round the globe getting stuck – as has happened over the past few weeks. Waves in the northern hemisphere jetstream placed the high pressure, hot and dry weather systems over western Europe, the Middle East, north Asia and North America.

lady holds cold water bottle against her haid on a sunny hot dayWeather is intrinsically variable - there will always be hotter and cooler periods - so although, by definition, heatwaves sit outside normal conditions, they are nothing new. A memorable heatwave took place in the UK during the summer of 1976, which saw exceptionally high temperatures for over two weeks, exacerbating a drought of many months standing. That year, however, did not see the same conditions occurring over many places in the northern hemisphere, as we have seen in 2018.

Is this heatwave due to climate change?

Nine of the ten warmest years have been recorded since 2005, so the background against which we judge these heatwaves is already warmer than it was in the past. This means that what used to be considered a high temperature and was extremely rare in the past, is now less unusual. One study suggests that heatwaves are over ten times more likely because of climate change. Further to this, other studies suggest that other extreme weather events, such as heavy rainfall, flash floods or strong wind storms, will also become more frequent, while cold snaps become less so.

Living in a 'Hothouse Earth'

a man taking a sunbath on a street bench on a sunny hot dayClimate 'tipping points' are theories that have been mooted for some time. The general idea is that some aspect of the climate system is pushed over a threshold, which brings about an irreversible new ‘normal’. For example, if warmer temperatures and drought harm plants in the Amazon rainforest, they will take up less carbon dioxide and transpire less water vapour, thereby enhancing both the greenhouse warming effect and the drought, which could potentially lead to a complete breakdown of the forest ecosystem. Another example is the melting of polar sea ice. As the ice melts, reflective white ice is replaced with dark ocean water that absorbs sunlight, and so enhancing the warming of the ocean and the atmosphere, circularly leading to more melting ice.

A new research paper by Steffen et al., which received much media attention last week under headlines of 'Hothouse Earth', speculates how some tipping points could act together such that the impacts of one enhances the possibility of another leading to a cascade effect. The authors suggested this could be triggered if global warming reaches two degrees above pre-industrial average temperatures, and that it would lead to a catastrophic new climate state referred to as 'Hothouse Earth'. None of this is a certainty, and the timescales and onsets unclear, but before you are too reassured by this, the story is certainly scientifically plausible.

Urgent need to act on climate change

This a timely reminder that the only way to avoid the worsening impacts of climate change is to reduce, as fast as possible, the emissions of greenhouse gases we create through energy production, transport, industry, agriculture and land-use.

Governments and authorities should be putting serious thought into how we can prepare for a range of extreme weather events that are predicted to become more frequent and more serious, as well as steer away from the Hothouse. Other outcomes of climate change could include food and water shortages, people migrating from uninhabitable areas and civil unrest, all of which can result in lost lives. I am one among many who would prefer to make the changes necessary to avoid living in this unsettling future.


Professors Joanna Haigh and Martin Siegert, Co-Directors of the Grantham Institute, have been speaking to the media about the study published today warning that climate change may become unstoppable within decades even if all countries met their targets to cut emissions. Speaking to the BBC's Victoria Derbyshire programme [at 10:34am], Professor Siegert said, "We’ve already done a lot of damage to the planet.... We must reduce our carbon dioxide emissions. The sooner we do it, the easier it will be. The longer we leave it, the more difficult and expensive it is". 

Professor Haigh spoke to BBC News at 6 [from 17m 17s] about the current heatwave and climate change. "As the globe warms, there will be more weather extremes – more heatwaves, more floods, more big storms. All of these are expected to come about as a result of climate change," she said.

Other coverage included BBC Breakfast, BBC News at 10, Radio Wales, ITN and the Jeremy Vine Show. Professor Siegert was also quoted in  The Guardian and France 24.

Imperial climate physicists Dr Michael Byrne and Dr Arnaud Czaja also gave interviews to media outlets, including London Live TV, BBC World News TV, and TRT World.

[Main image credit: georgeclerk]


Simon Levey

Simon Levey
Communications Division

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Tel: +44 (0)20 7594 5650
Email: s.levey@imperial.ac.uk

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