2,000 years of records show it’s getting hotter, faster



European heatwaves are part of a pattern of rapid global warming.
EPA/ABEL ALONSO

Ben Henley, University of Melbourne

New reconstructions of Earth’s temperature over the past 2,000 years, published today in Nature Geoscience, highlight the astonishing rate of the recent widespread warming of our planet.

We also now have a clearer picture of decade-to-decade temperature variations, and what drove those fluctuations before the industrial revolution took hold.

Contrary to previous theories that pre-industrial temperature changes in the last 2,000 years were due to variations in the Sun, our research found volcanoes were largely responsible. However, these effects are now dwarfed by modern, human-driven climate change.




Read more:
40 years ago, scientists predicted climate change. And hey, they were right


Reading the tree rings

Without networks of thermometers, ocean buoys and satellites to record temperature, we need other methods to reconstruct past climates. Luckily, nature has written the answers down for us. We just have to learn how to read them.

Corals, ice cores, tree rings, lake sediments, and ocean sediment cores provide a wealth of information about past conditions – this is called “proxy” data – and can be brought together to tell us about the global climate in the past.

Tree rings, corals and ice cores all provide ‘proxy data’ – information about changing temperatures over the centuries.
Simon Stankowski/Unsplash, CC BY

Teams of scientists around the world have spent many thousands of hours of field and laboratory work to collect and analyse samples, and ultimately publish and make available their data so other scientists can undertake further analysis.

Previously, our team, along with many other proxy experts, meticulously analysed and collated temperature-sensitive proxy data covering the last 2,000 years from around the world, creating the largest database of temperature-sensitive proxy data yet assembled. We then made all of the data publicly available in one place.

Astonishing consistency between reconstruction methods

With this unique dataset in hand, our team set about reconstructing past global temperature.

We scientists are notoriously sceptical of our own analysis. But what makes us more confident about our findings is when different methods applied to the same data yield the same result.

In this paper we applied seven different methods to reconstruct global temperature from our proxy network. We were astounded to find that the methods all gave remarkably similar results for multidecadal fluctuations – a very precise result considering the breadth of the methods used.

This gave us the confidence to delve further into what drove global temperature fluctuations on decadal timescales before the industrial revolution really took hold.

What happened before human-induced climate change?

Our study produces the clearest picture yet of Earth’s average temperature over the past two millennia. We also found that climate models performed very well in comparison, and they succeed in capturing the amount of natural variability in the climate system – the natural ups and downs in temperature from year-to-year and decade-to-decade.

Using climate models and reconstructions of external climate forcing, such as from volcanic eruptions and solar variability, we deduced that before the industrial revolution, global temperature fluctuations from decade to decade in the past 2,000 years were mainly controlled by aerosol forcing from major volcanic eruptions, not variations in the Sun’s output. Volcanic aerosols have a temporary cooling effect on the global climate. Following these temporary cooling periods our reconstructions show there is an increased probability of a temporary warming period due to the recovery from volcanic cooling.

Earlier this year One Nation leader Pauline Hanson suggested that volcanic eruptions may be responsible for the recent rise in atmospheric carbon dioxide levels.

Recent warming is far beyond natural variability

There are, of course, natural changes in Earth’s temperature from decade to decade, from century to century, and also on much longer timescales. With our new reconstructions were also able to quantify the rate of warming and cooling over the past 2,000 years. Comparing our reconstructions to recent worldwide instrumental data, we found that at no time in the last 2,000 years has the rate of warming been so high.

In statistical terms, rates of warming during all 51-year periods from the 1950s onwards exceed the 99th percentile of reconstructed pre-industrial 51 yr trends. If we look at timescales longer than 20 years, the probability that the largest warming trend occurred after 1850 greatly exceeds the values expected from chance alone. And, for trend lengths over 50 years, that probability swiftly approaches 100%. So what do all these stats mean? The strength of the recent warming is extraordinary. It is yet more evidence of human-induced warming of the planet.

But hasn’t there been natural climate change in the past?

Our understanding of past temperature variations of the Earth contributes to understanding such fundamental things as how life evolved, where our species came from, how our planet works and, now that humans have fundamentally altered it, how modern climate change will unfold.

We know that over millions of years, the movement of tectonic plates and interactions between the solid earth, the atmosphere and the ocean, have a slow effect on global temperature. On shorter (but still very long) timescales of tens to hundreds of thousands of years, our planet’s climate is gradually influenced by small variations in the geometry of the Earth and the Sun, for example, small wobbles and variations in the Earth’s tilt and orbit.

From the Last Glacial Maximum, about 26,000 years ago, when huge ice sheets covered large parts of the Northern Hemisphere landmass, Earth transitioned to a 12,000-year warm period, called the Holocene.




Read more:
Two centuries of continuous volcanic eruption may have triggered the end of the ice age


This was a time of relative stability in global temperature, apart from the temporary cooling effect of the odd volcano. With the development of human agriculture, our prosperity and population grew. Before the industrial revolution, Earth had not seen carbon dioxide concentrations above current levels for at least 2 million years.

Following the industrial revolution, warming commenced due to human activity. With a clearer picture of temperature variations over the past two millennia we now have a better understanding of the extraordinary nature of recent warming.

It is up to all of us to decide whether this is the kind of experiment we want to run on our planet.


I would like to gratefully acknowledge the leader of this study, Raphael Neukom, and my fellow co-authors from the PAGES 2k Consortium. We also owe the teams of proxy experts much gratitude. It is their generous contribution to science and to human knowledge that has allowed for this, and other palaeoclimate compilation and synthesis studies.The Conversation

Ben Henley, Research Fellow in Climate and Water Resources, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Why hot weather records continue to tumble worldwide


Andrew King, University of Melbourne

It sometimes feels like we get a lot of “record-breaking” weather. Whether it’s a heatwave in Europe or the “Angry Summer” in Australia, the past few years have seen temperature records tumble.

This is the case both locally – Sydney had its hottest year on record in 2016 – and globally, with the world’s hottest year in 2016 beating the record set only the year before.

Some of 2016’s heat was due to the strong El Niño. But much of it can be linked to climate change too.

We’re seeing more heat records and fewer cold records. In Australia there have been 12 times as many hot records as cold ones in the first 15 years of this century.

If we were living in a world without climate change, we would expect temperature records to be broken less often as the observational record grows longer. After all, if you only have five previous observations for annual temperatures then a record year isn’t too surprising, but after 100 years a new record is more notable.

In contrast, what we are seeing in the real world is more hot temperature records over time, rather than less. So if you think we’re seeing more record-breaking weather than we should, you’re right.

Why it’s happening

In my new open-access study published in the journal Earth’s Future, I outline a method for evaluating changes in the rate at which temperature records are being broken. I also use it to quantify the role of the human influence in this change.

To do it, I used climate models that represent the past and current climate with both human influences (greenhouse gas and aerosol emissions) and natural influences (solar and volcanic effects). I then compared these with models containing natural influences only.

Lots of hot records, fewer cold ones

Taking the example of global annual temperature records, we see far more record hot years in the models that include the human influences on the climate than in the ones without.

Crucially, only the models that include human influences can recreate the pattern of hot temperature records that were observed in reality over the past century or so.

Observed and model-simulated numbers of hot and cold global annual temperature records for 1861-2005. Observed numbers of record occurrences are shown as black circles with the model-simulated record numbers under human and natural influences (red box and whiskers) and natural influences only (orange box and whiskers) also shown. The central lines in the boxes represent the median; the boxes represent interquartile range.
Author provided

In contrast, when we look at cold records we don’t see the same difference. This is mainly because cold records were more likely to be broken early in the temperature series when there were fewer previous data. The earliest weather data comes from the late 19th century, when there was only a weak human effect on the climate relative to today. This means that there is less difference between my two groups of models.

In the models that include human influences on the climate, we see an increase in the number of global record hot years from the late 20th century onwards, whereas this increase isn’t seen in the model simulations without human influences. Major volcanic eruptions reduce the likelihood of record hot years globally in both groups of model simulations.

Projecting forward to 2100 under continued high greenhouse gas emissions, we see the chance of new global records continuing to rise, so that one in every two years, on average, would be a record-breaker.

Chance of record hot global annual temperatures in climate models with human and natural influences (red) and natural influences only (orange). Grey curve shows the statistical likelihood of a new hot record each year (100% in the first year, 50% in the second year, 33% in the third year, and so on). Grey vertical bars show the timing of major volcanic eruptions through the late-19th and 20th centuries.
Author provided

I also looked at specific events and how much climate change has increased the likelihood of a record being broken.

I used the examples of the record hot years of 2016 globally and 2014 in Central England. Both records were preceded by well over a century of temperature observations, so in a non-changing climate we would expect the chance of a record-breaking year to be less than 1%.

Instead, I found that the chance of setting a new record was increased by at least a factor of 30 relative to a stationary climate, for each of these records. This increased likelihood of record-breaking can be attributed to the human influence on the climate.

More records to come?

The fact that we’re setting so many new hot records, despite our lengthening observation record, is an indicator of climate change and it should be a concern to all of us.

The ConversationThe increased rate at which we are getting record hot temperatures is controlled by the speed of global warming, among other factors. To meet the Paris target of keeping global warming below 2℃ we will have to reduce our greenhouse gas emissions drastically. Besides keeping average global temperatures under control, this would also reduce the chance of temperature records continuing to tumble, both globally and locally.

Andrew King, Climate Extremes Research Fellow, University of Melbourne

This article was originally published on The Conversation. Read the original article.

Why the climate is more sensitive to carbon dioxide than weather records suggest



File 20170705 21578 1gwvogb
A new paper improves our estimate of the climate’s sensitivity to carbon dioxide.
NASA/Wikimedia Commons

Andrew Glikson, Australian National University

One of the key questions about climate change is the strength of the greenhouse effect. In scientific terms this is described as “climate sensitivity”. It’s defined as the amount Earth’s average temperature will ultimately rise in response to a doubling of atmospheric carbon dioxide levels.

Climate sensitivity has been hard to pin down accurately. Climate models give a range of 1.5-4.5℃ per doubling of CO₂, whereas historical weather observations suggest a smaller range of 1.5-3.0℃ per doubling of CO₂.

In a new study published in Science Advances, Cristian Proistosescu and Peter J. Huybers of Harvard University resolve this discrepancy, by showing that the models are likely to be right.

According to their statistical analysis, historical weather observations reveal only a portion of the planet’s full response to rising CO₂ levels. The true climate sensitivity will only become manifest on a time scale of centuries, due to effects that researchers call “slow climate feedbacks”.

Fast and slow

To understand this, it is important to know precisely what we mean when we talk about climate sensitivity. So-called “equilibrium climate sensitivity”, or slow climate feedbacks, refers to the ultimate consequence of climate response – in other words, the final effects and environmental consequences that a given greenhouse gas concentration will deliver.

These can include long-term climate feedback processes such as ice sheet disintegration with consequent changes in Earth’s surface reflection (albedo), changes to vegetation patterns, and the release of greenhouse gases such as methane from soils, tundra or ocean sediments. These processes can take place on time scales of centuries or more. As such they can only be predicted using climate models based on prehistoric data and paleoclimate evidence.

On the other hand, when greenhouse gas forcing rises at a rate as high as 2–3 parts per million (ppm) of CO₂ per year, as is the case during the past decade or so, the rate of slow feedback processes may be accelerated.

Measurements of atmosphere and marine changes made since the Industrial Revolution (when humans first began the mass release of greenhouse gases) capture mainly the direct warming effects of CO₂, as well as short-term feedbacks such as changes to water vapour and clouds.

A study led by climatologist James Hansen concluded that climate sensitivity is about 3℃ for a doubling of CO₂ when considering only short-term feedbacks. However, it’s potentially as high as 6℃ when considering a final equilibrium involving much of the West and East Antarctic ice melting, if and when global greenhouse levels transcend the 500-700ppm CO₂ range.

This illustrates the problem with using historical weather observations to estimate climate sensitivity – it assumes the response will be linear. In fact, there are factors in the future that can push the curve upwards and increase climate variability, including transient reversals that might interrupt long-term warming. Put simply, temperatures have not yet caught up with the rising greenhouse gas levels.

Prehistoric climate records for the Holocene (10,000-250 years ago), the end of the last ice age roughly 11,700 years ago, and earlier periods such as the Eemian (around 115,000-130,000 years ago) suggest equilibrium climate sensitivities as high as 7.1-8.7℃.

So far we have experienced about 1.1℃ of average global warming since the Industrial Revolution. Over this time atmospheric CO₂ levels have risen from 280ppm to 410ppm – and the equivalent of more than 450ppm after factoring in the effects of all the other greenhouse gases besides CO₂.

Estimate of climate forcing for 1750-2000.
Author provided

Crossing the threshold

Climate change is unlikely to proceed in a linear way. Instead, there is a range of potential thresholds, tipping points, and points of no return that can be crossed during either warming or transient short-lived cooling pauses followed by further warming.

The prehistoric records of the cycles between ice ages, namely intervening warmer “interglacial” periods, reveal several such events, such as the big freeze that suddenly took hold about 12,900 years ago, and the abrupt thaw about 8,200 years ago.

In the prehistoric record, sudden freezing events (called “stadial events”) consistently follow peak interglacial temperatures.

Such events could include the collapse of the Atlantic Mid-Ocean Circulation (AMOC), with consequent widespread freezing associated with influx of extensive ice melt from the Greenland and other polar ice sheets. The influx of cold ice-melt water would abort the warm salt-rich AMOC, leading to regional cooling such as is recorded following each temperature peak during previous interglacial periods.

Over the past few years cold water pools south of Greenland have indicated such cooling of the North Atlantic Ocean. The current rate of global warming could potentially trigger the AMOC to collapse.

A collapse of the AMOC, which climate “sceptics” would no doubt welcome as “evidence of global cooling”, would represent a highly disruptive transient event that would damage agriculture, particularly in the Northern Hemisphere. Because of the cumulative build-up of greenhouse gases in the atmosphere such a cool pause is bound to be followed by resumed heating, consistent with IPCC projections.

The growth in the cold water region south of Greenland, heralding a possible collapse of the Atlantic Mid-Ocean Circulation.
Author provided

Humanity’s release of greenhouse gases is unprecedented in speed and scale. But if we look far enough back in time we can get some clues as to what to expect. Around 56 million years ago, Earth experienced warming by 5-8℃ lasting several millennia, after a sudden release of methane-triggered feedbacks that caused the CO₂ level rise to around 1,800ppm.

The ConversationYet even that sudden rise of CO₂ levels was lower by a large factor than the current CO₂ rise rate of 2-3ppm per year. At this rate, unprecedented in Earth’s recorded history of the past 65 million years (with the exception of the consequences of asteroid impacts), the climate may be entering truly uncharted territory.

Andrew Glikson, Earth and paleo-climate scientist, Australian National University

This article was originally published on The Conversation. Read the original article.

A year of records: the human role in 2014’s wild weather


Mitchell Black, University of Melbourne; Andrew King, University of Melbourne, and David Karoly, University of Melbourne

Australia has just had its hottest October, and we can already say that human-caused climate change made this new record at least ten times more likely than it would otherwise have been.

But if we turn our eyes to the past, what role did climate change play in the broken records of 2014? Last year was the hottest on record worldwide, and came with its fair share of extremes.

As part of the annual extreme weather issue of the Bulletin of the American Meteorological Society released today, five papers by Australian authors including us, investigate the role of climate change in extreme weather in 2014.

Year of records

Australia was hit hard in 2014 (although perhaps not quite as hard as 2013, which was Australia’s hottest year ever).

The year started with a bang when the international spotlight fell on southeast Australia as Melbourne was baked by the infamous “Australian Open heatwave”. It led into 12 months that saw the country experience a 19-day heatwave in May, the hottest spring on record, and unusually hot weather in Brisbane during November, right in the middle of the G20 World Leaders Forum.

The Australian Open heatwave of 2014 saw several days above 40C in southern Australia.
Author provided

In August, a record high pressure system stalled to the south of Australia and brought some unusual winter weather, including severe frosts.

So did human-caused global warming play a role in this “weirding” of Australian weather?

Revealing the role of climate change

The annual extremes issue centres on one of the fastest developing areas in climate change research, the role of climate change in recent extreme weather events.

While the link between human activities and climate change has been firmly established for several decades, attributing a single event to human influence isn’t easy. This is because individual events may be the result of natural climate variation.

To get to the heart of how climate change is influencing these extreme events, scientists try to determine how much more likely individual extremes are as a result of climate change. Using climate models they compare the world of today with a parallel world without human greenhouse gas emissions.

These scenarios are often run on models thousands of times in an effort to recreate events that are of a similar scale. By comparing the results of modelled climates with and without human-produced greenhouse gases, researchers can determine how much more likely it is that an extreme weather event occurred as a result of human-caused global warming.

This approach is similar to the way epidemiologists investigate whether smoking increases the likelihood of lung cancer.

Interestingly, there was a significant citizen science role in three of the Australian peer-reviewed studies reported in the extremes issue. Using a large number of climate simulations run on thousands of home computers as part of the Weather@home project, the scientists were able to examine local-scale extreme events such as the January heatwave in Melbourne.

Citizen computing power has helped crunch the numbers and simulate climate extremes.
Weather@home

What we found

The first study, led by Mitchell Black, focused on the prolonged heatwave in southeast Australia in January 2014. During this event Adelaide recorded five consecutive days above 42°C (13–17 January) while Melbourne recorded four consecutive days above 41°C (14–17 January) during the Australian Open tennis tournament.

This study found that human influence very likely increased the chance of prolonged heatwaves in Adelaide by at least 16%. Meanwhile, the influence for Melbourne was less clear.

The second study, led by Andrew King, examined an extreme temperature event caught in the spotlight of international media attention – the unseasonably hot weather in Brisbane during the G20 summit in mid-November. While the hot temperatures were not record-breaking in Brisbane at this time, they were well above average.

This study found that human influence increased the likelihood of hot (above 34°C) and very hot (above 38°C) November days in Brisbane by at least 25% and 44%, respectively.

The third study, led by Michael Grose at CSIRO, examined the exceptionally high surface pressure to the south of Australia during August 2014. This was associated with severe frosts in southeast Australia, lowland snowfalls in parts of Tasmania, and reduced rainfall in the southern parts of both Australia and New Zealand.

The findings suggested that the likelihood of these pressure anomalies had roughly doubled due to human-induced climate change.

The remaining two studies published today used independent sets of climate model simulations.

The fourth study, led by Sarah Perkins-Kirkpatrick from the University of New South Wales, investigated the late-autumn heatwave (May 8-26) that resulted in Australian-averaged maximum temperatures being 2.52°C above the monthly average. Although this heat event occurred during the cooler months, events of this nature are important because they can affect agricultural productivity through changing crop cycles.

The study found that this kind of cool-season heatwave was 23 times more likely as a result of increased greenhouse gasses.

Pandora Hope from the Australian Bureau of Meteorology led the final study, which examined Australia’s hottest spring on record. The study concluded that the record heat would likely not have occurred without increases in atmospheric carbon dioxide over the last 50 years working in concert with anomalous atmospheric patterns.

Year on year, in the extremes issues and through various other investigations reported in the peer-reviewed literature, these attribution studies continue to show that climate change is no longer something that will occur in the future. The rise of human-caused global warming is here, now, and it is already causing changes to extreme weather events that we can see and feel.

The Conversation

Mitchell Black, PhD Candidate, University of Melbourne; Andrew King, Climate Extremes Research Fellow, University of Melbourne, and David Karoly, Professor of Atmospheric Science, University of Melbourne

This article was originally published on The Conversation. Read the original article.