2k Network Nature paper August 2016
Additional information to accompany:
"Early onset of industrial‐era warming across the oceans and continents"
Authors: Nerilie J. Abram, Helen V. McGregor, Jessica E. Tierney, Michael N. Evans, Nicholas P. McKay, Darrell S. Kaufman and the PAGES 2k Consortium*
(*See the full list of authors in the "Authors" tab below)
Vol. 536, pp. 411-415
Published online 24 August 2016
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Nerilie J. Abram
QEII Research Fellow (Australian Research Council)
Research School of Earth Sciences
Australian National University
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1. By undertaking a comprehensive assessment of climate information spanning the past 500 years, this study shows that human-induced climate warming commenced in many parts of the world as early as the 1830s.
2. Long climate simulations and past climate data are in good agreement on this early onset of greenhouse warming in most regions. An exception is the Antarctic, where past climate data and recent observations show that the development of widespread warming has been delayed, possibly because of the way ocean circulation in this region transports the surface warming signal away from the Antarctic.
3. Humans only caused small increases in the level of greenhouse gases in the atmosphere during the 1800s, compared with the much more rapid increases seen nowadays. But the early onset of warming detected in this study indicates that Earth's climate did respond in a measureable way to even these small human-induced changes in greenhouse gases during the 1800s.
4. This new evaluation of when greenhouse warming began is only possible because of the long view given by natural recorders of Earth's climate, such as corals, tree-rings and ice cores, as well as advances in supercomputing power that now enable climate models to simulate many hundreds of years.
5. The findings have implications for assessing how much humans have caused our climate to change. In the tropical oceans and the Arctic in particular anthropogenic climate change has been underway for around 180 years and has already caused the climate of these regions to move above the range of normal pre-industrial variability.
Q1: We've known for many decades already that Earth's climate is warming because of the rising levels of greenhouse gases in the atmosphere. What is new about this study?
A: Most studies of human-induced climate change have used instrumental records to study how quickly climate is warming across different parts of the world. Our study uses a collection of natural archives that preserve information about past temperatures over a much longer period, spanning the last 500 years, to ask the question: "When did the sustained warming trends that we’ve seen in the 20th and 21st Centuries first begin?" This is important because to determine how much we have altered the climate in different parts of the world we need to know how long our climate has been warming, as well as how quickly it has been warming.
The other new aspect of this study is that we have used records from the oceans as well as from land to assess how industrial-era climate warming developed across different parts of the world. The oceans cover two-thirds of the globe and are an important regulator of Earth's surface temperature because of their ability to move heat away from the surface and store it in deeper layers. Since the 1950s, the oceans are thought to have taken up ~90% of the excess heat trapped by increasing concentrations of greenhouse gases in the atmosphere (for example see the work of Levitus et al. 2012). Circulation of this heat into and out of different layers of the ocean can cause surface air temperature to have decadal intervals of rapid warming followed by intervals of less severe warming, such as the global warming slowdown that happened between 2001 and 2014 (e.g. as discussed in England et al. 2014). The oceans are fundamental to how our climate works, so it is very important that they are not overlooked when we study how our climate is changing.
Q2: What is the most important new conclusion from the study?
A: The most important finding of our study is that in some parts of the world, human-induced climate warming began as early as the 1830s. This shows that Earth's climate responded rapidly to even small increases in atmospheric greenhouse gas levels during the early parts of the Industrial Revolution. This early onset means that in some regions over 180 years of human-induced climate warming has already caused the climate to move above the range of natural variability in the centuries prior to the Industrial Revolution.
Q3: Would greenhouse warming have been noticeable to people during the 19th Century?
A: No. It is only by having the long record of anthropogenic warming into the 20th Century that we are able to see that the earliest stages of this ongoing warming trend began in the mid 19th Century. This sustained warming had to continue for many decades before it caused the climate to be changed significantly from its range of natural variability. For example, our analysis shows that in the Arctic, where warming began in the 1830s and has also been very rapid, it took roughly 100 years (1930s) for the industrial era warming signal to "emerge" above the range of normal year-to-year climate variability (see table 1 in the paper).
Q4: What is the evidence that the early warming signal is due to anthropogenic greenhouse gases?
A: Climate model simulations where only greenhouse gases are changed produce the same mid 19th Century onset of industrial-era warming as what we find in records of past climate from the tropical oceans and Northern Hemisphere continents. It is quite likely that the recovery of Earth's climate from the very large eruption of the Tambora volcano in 1815 (which caused, for example, the "year without summer" in historical accounts from Europe) also played a role in constraining the onset of industrial-era warming to a quite narrow window of time during the mid 19th Century. But our assessments of climate simulations showed that recovery of the climate from this volcanic cooling event wasn't required to explain the early onset of industrial-era warming.
The climate model simulations used in this study were part of the internationally coordinated effort that contributed towards the Fifth Assessment Report of the UN Intergovernmental Panel on Climate Change (IPCC) in 2013. These simulations apply all of the known natural and human factors that may have affected climate during the last millennium, including atmospheric greenhouse gases, the sun and volcanoes. The simulations are supplemented with additional experiments where each of these factors are applied individually. In the Northern Hemisphere and the tropical oceans, there is excellent agreement between the model simulations and the regional climate changes recorded by natural archives.
Q5: The increases in greenhouse gases during the 19th Century were very small compared with 20th Century changes. How can we detect a climate impact of this small perturbation during the 19th Century?
A: We are able to detect the early signals of industrial-era warming by using long datasets that begin well before and extend well after this climatic change point. In this study we look across the last 500 years. This gives the long view needed to detect the small but measurable climate response caused by the small increases in atmospheric greenhouse gas levels that were already underway in the 19th Century. In the 1830s, atmospheric CO2 levels were approximately 280 parts per million (ppm), and gradually increased to reach 295 ppm by the end of the 19th Century. For comparison, atmospheric CO2 is now at 403 ppm and increasing rapidly, so that it currently takes just seven years for the CO2 level to increase by 15 ppm.
The statistical method used in this study to objectively pinpoint when climate warming first began was also an important advance. Climate warming has accelerated as the increases in greenhouse gases in our atmosphere have become more rapid. Our statistical approach was adaptable to the changing rates of climate warming through time because it didn’t rely on looking just at the average linear change in temperature from the earliest stages of warming through to today.
Q6: What are the major regional differences in the onset of industrial-era warming that were found in this study?
A: Industrial era warming first began in the tropical oceans and over landmasses in the Northern Hemisphere during the mid 19th Century. In some areas of the Southern Hemisphere, warming may not have started until about 50 years later. The Antarctic continent as a whole doesn't yet appear to show evidence for significant climate warming, although some regions of Antarctica have warmed rapidly in the last 50 years (see also Q8 and Q9).
Q7: Why is anthropogenic climate warming not globally synchronous?
A: In the same way that not all parts of the world are warming at the same speed, the warming did not start at the same time everywhere. For example, circulation of the oceans can cause sub-surface water that has not yet been affected by anthropogenic warming to be brought to the surface, effectively delaying the onset of warming in those regions. However, most parts of the tropical oceans have a stable surface layer and in these areas warming developed at the same time and has progressed as quickly as the atmospheric warming that has taken place over land in the Northern Hemisphere.
Q8: Why was the onset of warming in the Southern Hemisphere delayed relative to the Northern Hemisphere?
A: We don't fully understand this yet, but it is likely due to the different configuration of the oceans and continents in the Southern Hemisphere, which causes some important differences in how ocean and atmospheric circulation regulate the climate here. There is also a lot less data available from the Southern Hemisphere with which to assess the onset of industrial-era warming, adding uncertainty to the assertion that there was a delayed onset of warming in the Southern Hemisphere. Developing a more comprehensive understanding of how the Southern Hemisphere has responded to anthropogenic greenhouse warming is an important line of investigation for future research.
Q9: Why is Antarctica not yet warming?
A: The Antarctic continent as a whole doesn't yet show evidence for significant industrial-era warming, either based on long ice core records or more recent observational records. But some regions of the Antarctic, particularly West Antarctica and the Antarctic Peninsula, have been warming rapidly since the mid 20th Century and losing ice mass that is contributing to sea-level rise (see Jones et al. 2016 for a review). The lack of continent-scale warming over Antarctica is probably related to the thermal isolation of Antarctica. The westerly winds that circle the Southern Ocean around Antarctica keep warm air masses from the mid-latitudes from reaching the Antarctic continent, and ozone depletion and rising greenhouse gases have been causing this wind barrier to get stronger. The circumpolar current, which flows around Antarctica in the Southern Ocean, also causes any surface warming of the ocean to be moved away from Antarctica and replaced with cold sub-surface water that hasn't yet seen the effects of greenhouse warming. It has been suggested that this ocean circulation process could delay the development of warming of the Southern Ocean around Antarctica by hundreds of years (Armour et al. 2016).
Q10: Why do observations and climate-model simulations appear to disagree in the Southern Hemisphere?
A: This is an unresolved problem that has only recently become apparent now that we have multi-century climate model simulations and Southern Hemisphere climate reconstructions to compare. Disagreement between the data and the models has a number of potential causes. Firstly, it may simply be a consequence of natural (unforced) variability within the climate system, which is known to be large in high-latitude parts of the Southern Hemisphere. Secondly, it could be caused by certain processes, such as sea ice or fine-scale ocean circulation, that are important in the Southern Hemisphere climate system but are not well characterised within current climate models. Finally, data on past climates is scarce for many Southern Hemisphere regions so there is greater uncertainty in our knowledge of how Southern Hemisphere climate behaved in the past. Resolving the apparent differences between data and models in the Southern Hemisphere will be an important goal of future research.
Q11: What implications do the findings have for responding to anthropogenic greenhouse warming?
A: The early onset of warming means that, in some areas of the world, human-induced climate change has been taking place for ~180 years and has already caused the average climate to rise above the level of natural variability in pre-industrial times. This is important to consider when assessing the extent to which anthropogenic climate change has altered the environmental conditions that ecosystems are adapted to, and to broader discussions on the threshold of anthropogenic climate warming that is considered acceptable or safe.
The early onset of industrial-era warming also demonstrates that Earth's climate responds in a rapid and measureable way to even small changes in greenhouse gas levels. It will be important for future climate change mitigation efforts to discover if efforts to reduce or reverse greenhouse gas emissions could similarly result in rapid climate "paybacks" in some regions.
Q12: Some scientists believe that humans have been altering the climate for thousands of years. How does this research relate to these ideas?
A: It is thought that the development of farming thousands of years ago could explain the gradual changes of CO2 (carbon dioxide) and CH4 (methane) levels in Earth's atmosphere over the last ~8,000 years (e.g. Lewis and Maslin 2014; Ruddiman et al. 2016). Our study focuses only on the last 500 years to study the significant and sustained climate warming caused by burning of fossil fuels since the Industrial Revolution. The speed at which CO2 levels have increased in the atmosphere since the Industrial Revolution (~120 ppm increase in the last 200 years) is much greater than the changes that may have been caused by early agriculture (~20 ppm increase in the 8000 years prior to the Industrial Revolution).
Amour et al. 2016, Nature Geoscience, doi: 10.1038/ngeo2731.
England et al. 2014, Nature Climate Change, doi: 10.1038/nclimate2106.
Jones et al. 2016 (in press), Nature Climate Change. Assessing recent trends in high‐latitude Southern Hemisphere surface climate.
Levitus et al. 2012, Geophysical Research Letters, doi: 10.1029/2012gl051106.
Lewis and Maslin 2014, Nature, doi: 10.1038/nature14258.
Ruddiman et al. 2016, Reviews of Geophysics, doi: 2015RG000503.
Figure 1: Onset of industrial-era warming in regional temperature reconstructions. Colored lines show regional temperature reconstructions since 1500 AD with a 15-year (thin gray lines) and 50-yr (thick gray lines) smoothing. The red vertical bars represent the time when sustained, significant industrial-era warming is determined to have first begun. 1°C scale bar denotes the y-axis scale of each regional temperature reconstruction.
Climate scientists from across the world, who carried out a comprehensive analysis of the climate information from natural archives and long model simulations.
Images feature Nerilie Abram (Australian National University), Helen McGregor (University of Wollongong), Michael Evans (University of Maryland) and Mark Curran (Australian Antarctic Division)
Natural climate records from across the oceans and continents show that human-induced greenhouse warming first developed in the mid 19th Century.
Layers of ice that have built up year-after year to form ice sheets over Antarctica and Greenland provide a way of reconstructing Earth’s past temperature.
Cave samples record changes in Earth’s climate. Permission to use this image has been given by Christopher Maupin and Meaghan Gorman.
Corals, such as this one at Rowley Shoals, west of Broome in Western Australia, record detailed climate histories in a similar way to the yearly growth rings of trees.
Photo courtesy of Eric Matson, Australian Institute of Marine Science
A high-resolution version of this image is available on request from the British Antarctic Survey, at the following link:
Picture by Henry Gastineau of ironworks in South Wales during the Industrial Revolution c.1820
> Nature warming trends animation by Abram et al (.mov, 50 seconds): Century-scale temperature trends for the continents and tropical oceans over the last 500 years. Colours show the regional 100-year temperature trends, for every year since 1500CE. Indicator bar below the map shows the time-span of the 100-year trends. Non-significant trends are masked in grey.
The 2k Network is made up of nine regional working groups. Each regional group collects and processes the best time series and spatial reconstructions of important state variables of the climate system (e.g. surface and 500 hPa geopotential, temperature and precipitation).
> Africa2k (African climate of the last 2 millennia)
> Antarctica2k (Antarctic climate of the last 2 millennia)
> Arctic2k (Arctic climate of the last 2 millennia)
> Asia2k (Asian climate of the last 2 millennia)
> Aus2k (Australasian climate of the last 2 millennia)
> Euro-Med2k (European and Mediterranean climate of the last 2 millennia)
> LOTRED-SA (Long-Term Climate Reconstruction and Dynamics of South America)
> NAmerica2k (North American climate of the last 2 millennia)
> Ocean2k (Marine climate of the last 2 millennia)
Past Global Changes (PAGES) was established in 1991 to facilitate international research into understanding past changes in the Earth system to improve projections of future climate and environment, and inform strategies for sustainability. It receives funding mainly from the Swiss and US National Science Foundations. PAGES is a core project of Future Earth and a scientific partner of WCRP.
More at: http://pastglobalchanges.org
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