Current climate warming is the worst for 2,000 years

| July 26, 2019
The speed and regional extent of global changes in temperature towards the end of the 20th century is far greater than other climate fluctuations over the the past 2,000 years. These findings are reported in two papers in Nature and Nature Geoscience that examine temperature trends over the past two millennia.

Climate variability over the past two millennia has been subject to debate. Notable periods include the Medieval Climate Anomaly, the Little Ice Age, and rapid warming over the past 150 years in response to the human influence on climate. Determining the extent of these periods, along with gaining a clear understanding of the factors that caused variability in the past, has been challenging.

Raphael Neukom and colleagues assessed the global patterns of climate variability during the Common Era, using data compiled from nearly 700 proxy records of temperature changes. In their Nature paper, they report that before the 20th century, climate epochs did not occur simultaneously across the globe as previously thought.

For example, the coldest temperatures over the Common Era occurred in central and eastern Pacific regions in the 15th century, in northwestern Europe and southeastern North America in the 17th century, and elsewhere during the 19th century.

Similarly, no pre-industrial period experienced globally coherent, long-term warmth. By contrast, the warmest period during the Common Era occurred in its final decades for more than 98% of the globe.

In their Nature Geoscience paper, Neukom and co-authors examine rates of surface warming and driving forces averaged over decades. Their analyses reveal that rates of warming over periods of at least 20 years were fastest during the later 20th century.

Pre-industrial fluctuations were primarily driven by volcanic activity. Agreement between reconstructions and simulations suggests that climate forecasts for the next few decades may be realistic, the authors propose.

The role of volcanic eruptions in the early 19th century climate is probed further in another related Nature Geoscience paper by Stefan Brönnimann and colleagues. They report that a cluster of eruptions was followed by sustained cooling and climatic upheaval, including droughts in Africa and weak monsoons.

The recovery from this cooling resembled transited into a period that coincided with early influences from the industrial revolution, which makes it more difficult to determine the relative impacts of these factors.

Together, these studies help to paint a detailed picture of how climate variability changed between the pre-industrial period and the 20th century.

Expert Reaction

Ben Henley is a Research Fellow in the School of Earth Sciences at the University of Melbourne and an author on the paper.

“The findings give us the clearest picture yet of the speed and global extent of modern climate change relative to the pre-industrial climate of the last 2000 years.

Our study shows that the most rapid warming of the past 2000 years has occurred during, and since, the second half of the 20th century, highlighting the extraordinary character of current climate change, due mostly to human emissions of heat-trapping gases.

We humans are responsible for immense changes to the global climate. It is up to the global community to determine if this is what we want to be doing to our planet. Australia has an important part to play in that global community.”

Fiona Armstrong is Founder and Executive Director of the Climate and Health Alliance, a founding director of CLIMARTE: Arts for a Safe Climate, a Fellow of the Centre for Policy Development, and an Associate of the Melbourne Sustainable Societies Institute at the University of Melbourne.

“It is significant that this study has shown that such prolonged warming, over 98 per cent of the globe, has never occurred previously. 

This global heating driven by human activity poses extremely dangerous risks to the health of people in every part of the world.

Our recent summer in Australia saw the most extraordinary extremes, with temperatures in many parts of Australia much hotter than usual and heatwaves lasting longer than usual. Many locations, including Adelaide, exceeded previous records by large margins.

This is leading to severe bushfires, including in once wet Tasmania, which experienced its driest ever January.

The fish kill in the Murray Darling is an example of what happens to other species as global heating affects ecosystems, but too little attention is being paid to the implications for human health and labour productivity.

Heatwaves are leading to premature deaths, and affect not only vulnerable members of the community, like the very young and the very old, but also can be deadly for people exposed to severe heat at work.  

Humans actually have a very narrow temperature range that they can tolerate, and impacts can range from headaches to multiple organ failure and collapse.

This research is a wake-up call that we are heating the planet, and we will suffer increasing impacts if we fail to take action to reduce emissions very very soon.”

Professor Ian Simmonds works at the School of Earth Sciences at the University of Melbourne.

“The five hottest global mean temperatures since 1850 have all occurred in the last five years. The odds against this occurring by chance are astronomical and indicates that large changes are afoot.

On a longer timescale, Neukom et al. and the PAGES 2k Consortium have carefully and painstakingly examined a vast array of temperature records and its proxies over the last 2000 years.

They found the largest warming on multi-decadal timescales to have occurred during the second half of the twentieth century, highlighting the remarkable character of the warming in recent decades, and the role of increasing greenhouse gases. 

One of the key factors associated with these changes is the behaviour of the Atlantic Meridional Overturning Circulation (AMOC) (which is part of the ‘Global Ocean Conveyor Belt’).

This circulation has weakened by 15 per cent since 1950, and that weakening has been linked with high summer temperatures over Europe and the extreme heat events of 2015 and 2003. Consistent with the results of Neukom et al. and the PAGES 2k Consortium climate model recent studies indicate that the AMOC demonstrated little overall change in the 1000-year period up to about 1850.”

Dr Paul Read is Senior Research Fellow at the Monash Sustainability Institute at Monash University and a Co-Director of the National Centre for Research in Bushfire and Arson.

“These two new climate papers come at an opportune time from the world’s leading scientific publication, Nature.   

People should bear in mind that these papers don’t stretch back as far as we can measure but they get much closer to understanding our more recent past – what’s been happening over the past 2000 years.

To my reading, there are four main messages.

The first is the headline that recent warming far outstrips anything we’ve seen in the past 2000 years, even when taking into account the Medieval Anomaly, Little Ice Age, and others that sceptics have been so fond of citing in the past.

Yes, we can measure the natural fluctuations stretching back much longer, proxied back to half a million years and more, depending on how accurate you want to be, but what we’re witnessing today is over and above what these fluctuations would predict.

As of May 2013, we hadn’t seen this density of atmospheric carbon emissions for more than 800,000 years. It’s consistent with what we’re seeing in natural disasters and bushfires around the world.

We’ve got bushfires in high latitude nations like Alaska, Russia and Greenland, and, even yesterday, NSW had to start considering the probability of mid-winter fires in Australia! That’s just one effect.

Three more take-home messages arise from these papers.

Note in the first paper led by Raphael Neukom that fully 700 different measures of temperature and its proxies have been measured and they show that the parts of the globe have cooled or heated at different times.

This makes sense in a complex system like climate – warming and cooling has been all over the place. Except now.  The only time the globe has warmed across 98 per cent of its surface is, according to Neukom’s team, and this is supported by other studies, pretty much now – the past 40 years.  

Antarctica will be the last to fall and this will be the first time in at least 2000 years that the entire world went in the same climatic direction.  

Number three headline is this – even when the global variability is taken into account climate modelling has properly predicted the overall outcome. This means we need to take future climate modelling very, very seriously.  

The fourth and final headline comes from the paper led by Stefan Bronnimann.

This is an interesting one because it suggests caution, perhaps even a slice of optimism when using the pre-industrial period as our baseline for the Paris Agreement.

Their work suggests we had an unusual period of volcanic activity in the early 1800s which cooled the Earth rather than heating it.

I’ve had people ask me in the past whether volcanic activity might have influenced the models and I’ve pointed out that in normal years volcanic activity is responsible for minuscule amounts of emissions.

What this new information means is that we had a burst of 28 eruptions that meant true anthropogenic warming might not have started until the late 1800s, maybe even the early 1900s.

Overall we need to take our collective climate future seriously and we must use as much information as we can to model the likely impact.”

Dr Nerilie Abram is from the Research School of Earth Sciences at The Australian National University.

“There is absolutely no doubt that humans are fundamentally changing Earth’s climate.

These studies are the most comprehensive assessment of Earth’s climate over the past 2000 years and they show just how unusual the climate that we live in today is.

The temperature on Earth now, the speed with which it is increasing and the fact that this warming is affecting virtually every part of the globe are all unprecedented in the last 2000 years, and most likely in the whole of human history.

And this means that unless we drastically curb greenhouse gas emissions, then heatwaves, like the extreme summer in Australia last year and the events unfolding across Europe and North America at the moment, will become more and more commonplace in the future making it harder for people and ecosystems to keep pace with adapting to climate change.”

Associate Professor Pete Strutton is from the Institute for Marine and Antarctic Studies at the University of Tasmania and the ARC Centre of Excellence for Climate System Science.

“The focus of these papers is the variability in global temperatures over the last 2000 years. This work represents a large community effort that collated and rigorously analysed data from glaciers, lake sediments, tree rings, marine shells, corals and computer climate models.

Each of the papers targets a specific aspect of the climate of the last 2000 years, but there is a common theme: Earth’s climate has seen considerable variability over the last 2000 years.

Until about 150 years ago, that variability was more regional than global* and influenced mostly by processes such as volcanic eruptions. Human-caused warming is unprecedented in both its magnitude and its global extent. The climate models used in these studies were able to reproduce the observations, which reinforces our confidence in their future projections.”

“*’more regional than global’ just means that certain areas were affected, rather than the whole planet. To quote one of the papers: ‘In particular, we find that the coldest epoch of the last millennium—the putative Little Ice Age—is most likely to have experienced the coldest temperatures during the fifteenth century in the central and eastern Pacific Ocean, during the seventeenth century in northwestern Europe and southeastern North America, and during the mid-nineteenth century over most of the remaining regions.”

Dr Liam Wagner is a Lecturer in Economics specialising in energy economics and policy at Griffith University.

“The rate at which the climate is warming is becoming more alarmingly clear as we see the consequences of failing to mitigate carbon dioxide emissions. While modernity requires more energy to grow and survive, the need to rapidly transform our energy sector into a CO2 neutral renewable energy system is even more urgent.
It is now vital that the energy sector reduce emissions to protect against climate change and to provide a sustainable basis for economic development and poverty alleviation. Very hard choices will have to be made to achieve the UN ‘sustainable development’ goals which will become increasingly more difficult the world becomes warmer and inhospitable.”

Dr Liz Hanna is Chair of the Environmental Health Working Group at the World Federation of Public Health Associations, and an Honorary Senior Fellow at the Climate Change Institute in The Australian National University.

“Australia needs to take note. We now have very clear evidence that global warming is not only with us now, it is accelerating, and it is a global phenomenon, not mere local climatic variability. We know this trend will continue as global emissions continue to climb.

The northern hemisphere 2019 summer is breaking new records in Europe, the sub-continent, Asia and across the United States, exposing hundreds of millions to lethal heat exposures. 

The accompanying droughts and power outages dramatically increase human health risks by depriving people access to life-saving water and cooling.

Cities in the USA are declaring “Heat Emergencies”, yet despite increasing heat events, Australia’s preparedness remains woefully inadequate following decades of politicising and wilfully ignoring the sound scientific evidence.

Australia’s huge geographical size delivers heat exposure threats ranging from extreme-dry-heat to super-hot-with-high-humidity, so the responses must be tailored to the physiological threats.

On a national level, there is a need for a massive public education program akin to the Life Be In It campaigns so that ALL Australians understand heat risks, recognise the symptoms and know the management of heat stress. Importantly, measures to protect against heat stress must be common knowledge, even among children.

Australia needs to re-examine legislation frameworks to enshrine the duty of care to provide safe environments, at work, at play and in the community. Response capacity of Australia’s health and emergency sectors has failed previous heatwaves as victims wait 8 hours for an ambulance. Meanwhile, the health workforce still receives no heat management training. 

We have been warned. Heat Emergencies are barrelling towards us and Australia can no longer pretend this is not happening. It is quite simple, we either prepare or die.”

Ian Lowe is Emeritus professor of science, technology and society at Griffith University, Qld and former President of the Australian Conservation Foundation.

“This sophisticated new study uses a variety of methods to reconstruct the global temperature record for the last 2000 years.

It confirms “the extraordinary rate” of recent increases in global temperature and confirms that it is virtually certain that this warming is being caused by human activity. The study also shows that the models being used produce reasonable estimates of temperature increase, “further justifying the use of such models as a basis for regional-to-global scale simulations” of the future impacts of putting more greenhouse gases into the air. 

This surely means that it is now indefensible to behave as if slowing climate change is a luxury, an optional extra if it doesn’t slow economic growth.

More than 700 local authorities and several countries have accepted that we have “a climate emergency” that demands urgent and concerted action. So opening new coal provinces like the Galilee Basin, or expanding exports of other fossil fuels such as LNG, are acts of criminal irresponsibility.

As well as curbing exports of fossil fuels and cleaning up electricity supply by investing in renewables, we desperately need a coordinated approach to our release of greenhouse gases, with particular attention to transport, agriculture and manufacturing.”