In a new article in the journal Nature, Stephen Rintoul and colleagues present two very different visions of Antarctica’s future, from the perspective of an observer looking back from 2070. In one vision, humanity continues to exploit Earth’s natural resources (such as fossils fuels) and does little to protect the environment, and in the other, there is a global movement towards conservation. The article shows how Antarctica will change over the next 50 years, should either of these two situations occur.
Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula
This article is a summary of the following paper:
Davies, B.J., Golledge, N.R., Glasser, N.F., Carrivick, J.L., Ligtenberg, S.R.M., Barrand, N.E., van den Broeke, M.R,. Hambrey, M.J., and Smellie, J.L., 2014. Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula. Nature Climate Change. DOI: 10.1038/nclimate2369
Read the Press Release.
Climate change on the Antarctic Peninsula
It’s getting snowier on the Antarctic Peninsula. Glaciers are thickening in their accumulation centres, but are melting more and thinning in their lower portions. Snowfall is forecast to continue to increase over the next couple of decades, so what does this mean for glaciers on the Antarctic Peninsula? Will the increase in snow offset glacier melt?
We studied this phenomenon on James Ross Island, northern Antarctic Peninsula. Here, summer temperatures can reach above 0°C and glaciers are melting strongly. In 1995 AD, Prince Gustav Ice Shelf collapsed, which resulted in glacier acceleration, recession and thinning, which continues to this day.
We carried out extensive fieldwork on James Ross Island to map and analyse the changes to a glacier, which is currently 4km long, over the past 10,000 years. We used a combination of glacier and climate modelling, glacial geology and ice-core data. We found that small glaciers that end on land around the Antarctic Peninsula are highly vulnerable to slight changes in air temperature. Over the next few decades, they will be smaller than at any point during the last 10,000 years. Just small increases in air temperature increased melting so much that even large amounts of extra snowfall could not prevent glacier recession. These small glaciers around the Antarctic Peninsula are likely to contribute most to rising sea levels over the coming decades, because they can respond quickly to climate change.
Glacier fluctuations on James Ross Island
James Ross Island is well studied, being reasonably accessible by ship. There is a high resolution ice core, taken in 2007 AD, available from the summit of the Mount Haddington Ice Cap. The Ulu Peninsula is one of the largest ice-free areas in the region, which makes it ideal for glacial geological studies of past ice-sheet extent. Here, a large boulder train extends from Glacier IJR45 towards Brandy Bay, where it aligns with a large moraine. This moraine overlies reworked marine sediments with shells, which have been dated to ~5000 years ago. Because the shells are underneath the moraine, this suggests that the readvance that deposited the boulder train and the moraine occurred AFTER ~5000 years ago.
The ice-core temperature records from Mulvaney et al. (2012) tell us that the air temperatures on James Ross Island 5000 years ago were about 0.5°C warmer than today. Prince Gustav Ice Shelf was absent from ~6000 to 2000 years ago, during this relatively warm period. Ice shelf absence is usually indicative of strong surface melt.
Warm air usually carries more water, so a readvance may have occurred because there may have been more precipitation at this time. People have therefore hypothesised that the glacier readvanced during a period that was WARMER but WETTER than today, in response to increased snowfall. This behaviour is contrary to that observed today, with glacier recession during a period of warming and ice-shelf collapse. This readvance is therefore a good analogue for future glacier behaviour in this sensitive region.
Glacier modelling experiments
We used a computer numerical model to test these hypotheses of glacier behaviour. The glacier flowline model was calibrated and tested using the temperature and precipitation ice-core record over the last 160 years. The glacier flowline model (along line A-B in the Ulu Peninsula map) was calibrated and tested using the temperature and precipitation ice-core record over the last 160 years. The modelled glacier replicated rates of recession from 1970 AD to present, and finished in 2006 with a similar geometry and velocity to the observed glacier in 2006 AD, thus increasing confidence in our model.
We conducted a number of sensitivity experiments on the glacier model, including precipitation (snowfall), temperature, how easily snow and ice melt (snow and ice degree day factors), and how much the glacier ice deforms and slides (flow enhancement coefficient, sliding factor). The results are shown in the plots below. You can see that for a small decrease in temperature there is a very large increase in glacier length and volume, but there are only small changes for a +20% to -20% change in precipitation, degree day factors or the ice deformation factor.
The profiles below show just how different the length changes are between temperature and precipitation sensitivity experiments.
The next stage was to use the Holocene climate record to drive the glacier through the last 10,000 years. Snowfall data are unavailable for this length of time. At first, we kept precipitation constant at modern values (0.65 m per year). This resulted in a fairly stable glacier for most of the Holocene, with a small retreat from 6000-2000 years ago, when Prince Gustav Ice Shelf was absent, and a large readvance out to the bay and Brandy Bay Moraine in the last millennium.
To test the hypothesis that it was a warmer but WETTER climate that forced a readvance 5000 years ago, we varied precipitation with temperature. We varied it by 5%, 7.3%, 15%, 20% and 100%, so that for every 1°C of warming, it became 5%, 7.3% (etc.) wetter. We were feeding the glacier during warm periods and starving it during cool periods.
You can see from the figure (b) above that all this did was successively dampen the glacier’s response to cooling and warming. The readvance occurred at the same time, but was smaller according to amount of precipitation was reduced by. You can see this happening in profile view in the animation below.
These modelling experiments show that the glaciers on Ulu Peninsula remained largely stable for most of the Holocene, with likely recession during the period of ice-shelf collapse (6000-2000 years ago), and a large readvance during climatic cooling 1500-300 years ago, when the ice-shelf reformed. The glacier then receded rapidly in response to warming temperatures. The ice shelf collapsed again in 1995 AD, and the glacier continues to retreat today. This modelled behaviour is therefore more in line with current glacier observations.
We surmise that the readvance occurred during the Neoglacial, or “Little Ice Age” period, and not during the warmer period ~5000 years ago. Evidence for this is patchy, and there are few terrestrial records of ice advance at this time. Our study is the first to convincingly show glacier advance during a period of strong cooling during the last millennium. This research also suggests that, rather than being more extensive during warm periods in the past, glacier minima similar to present have occurred multiple times during the last 10,000 years.
To assess the significance of these findings within the context of future climate change, we performed simulations from 1980 to 2200 AD, with climate outputs from regional climate models. All four scenarios used predict temperature rises over the region for the next two hundred years, but projections of snowfall vary.
All four experiments predicted a strong decrease in glacier volume over the next two centuries. Glacier volume reaches minima not experienced at any point in the last 10,000 years within the next century, and the glacier has almost disappeared after 200 years.
Summary and conclusions
Glacier IJR45 is typical of many of the small, land-terminating glaciers peripheral to the Antarctic Peninsula, where surface melting is strongly controlled by air temperature and the length of the melt season. Both of these are projected to increase over coming decades, and summer melting will become increasingly important. These glaciers will therefore contribute significantly to sea level rise over coming decades to centuries. These processes are likely to be representative of regional glaciers.
Our main conclusions are:
- Glacier modelling, spanning a range of past, present and future time intervals, shows that Glacier IJR45 has a high sensitivity to temperature and is less sensitive to precipitation.
- Glacier advance during past and future warm periods is unlikely. Increases in precipitation on the Antarctic Peninsula will not offset glacier melting and recession.
- The most recent advance of the glacier likely occurred during the last millennium, peaking at ~300 years ago, during a period of strong cooling.
- The currently observed trends of glacier melting, thinning and recession around the Antarctic Peninsula will continue over coming decades to centuries, with glacier minima not experienced at any time over the last 10,000 years occurring within the next century.
Please read the original article and cite as:
Davies, B.J., Golledge, N.R., Glasser, N.F., Carrivick, J.L., Ligtenberg, S.R.M., Barrand, N.E., van den Broeke, M.R,. Hambrey, M.J., and Smellie, J.L., 2014. Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula. Nature Climate Change. DOI: 10.1038/nclimate2369
- Climate change
- Glaciers and climate change
- Antarctic Peninsula glacier change
- Glacier mass balance
- Ice shelf collapse
- Prince Gustav Ice Shelf
- Glacier modelling
- Ice-cored moraines on Ulu Peninsula, James Ross Island
- Sediment-landform assemblages on Ulu Peninsula, James Ross Island
- Palaeo ice sheet reconstruction
Coverage in the news
Why use ice cores?
Ice sheets have one particularly special property. They allow us to go back in time and to sample accumulation, air temperature and air chemistry from another time. Ice core records allow us to generate continuous reconstructions of past climate, going back at least 800,000 years. By looking at past concentrations of greenhouse gasses in layers in ice cores, scientists can calculate how modern amounts of carbon dioxide and methane compare to those of the past, and, essentially, compare past concentrations of greenhouse gasses to temperature.
Ice coring has been around since the 1950s. Ice cores have been drilled in ice sheets worldwide, but notably in Greenland and Antarctica[4, 5]. High rates of snow accumulation provide excellent time resolution, and bubbles in the ice core preserve actual samples of the world’s ancient atmosphere. Through analysis of ice cores, scientists learn about glacial-interglacial cycles, changing atmospheric carbon dioxide levels, and climate stability over the last 10,000 years. Many ice cores have been drilled in Antarctica.
How do ice cores work?
The large Greenland and Antarctic ice sheets have huge, high plateaux where snow accumulates in an ordered fashion. Slow ice flow at the centre of these ice sheets (near the ice divide) means that the stratigraphy of the snow and ice is preserved. Drilling a vertical hole through this ice involves a serious effort involving many scientists and technicians, and usually involves a static field camp for a prolonged period of time.
Shallow ice cores (100-200 m long) are easier to collect and can cover up to a few hundred years of accumulation, depending on accumulation rates. Deeper cores require more equipment, and the borehole must be filled with drill fluid to keep it open. The drill fluid used is normally a petroleum-derived liquid like kerosene. It must have a suitable freezing point and viscosity. Collecting the deepest ice cores (up to 3000 m) requires a (semi)permanent scientific camp and a long, multi-year campaign.
Layers in the ice
If we want to reconstruct past air temperatures, one of the most critical parameters is the age of the ice being analysed. Fortunately, ice cores preserve annual layers, making it simple to date the ice. Seasonal differences in the snow properties create layers – just like rings in trees. Unfortunately, annual layers become harder to see deeper in the ice core. Other ways of dating ice cores include geochemisty, layers of ash (tephra), electrical conductivity, and using numerical flow models to understand age-depth relationships.
Although radiometric dating of ice cores has been difficult, Uranium has been used to date the Dome C ice core from Antarctica. Dust is present in ice cores, and it contains Uranium. The decay of 238U to 234U from dust in the ice matrix can be used to provide an additional core chronology.
Information from ice cores
The thickness of the annual layers in ice cores can be used to derive a precipitation rate (after correcting for thinning by glacier flow). Past precipitation rates are an important palaeoenvironmental indicator, often correlated to climate change, and it’s an essential parameter for many past climate studies or numerical glacier simulations.
Ice cores provide us with lots of information beyond bubbles of gas in the ice. For example, melt layers are related to summer temperatures. More melt layers indicate warmer summer air temperatures. Melt layers are formed when the surface snow melts, releasing water to percolate down through the snow pack. They form bubble-free ice layers, visible in the ice core. The distribution of melt layers through time is a function of the past climate, and has been used, for example, to show increased melting in the Twentieth Century around the NE Antarctic Peninsula.
Past air temperatures
It is possible to discern past air temperatures from ice cores. This can be related directly to concentrations of carbon dioxide, methane and other greenhouse gasses preserved in the ice. Snow precipitation over Antarctica is made mostly of H216O molecules (99.7%). There are also rarer stable isotopes: H218O (0.2%) and HD16O (0.03%) (D is Deuterium, or 2H). Isotopic concentrations are expressed in per mil δ units (δD and δ18O) with respect to Vienna Standard Mean Ocean Water (V-SMOW). Past precipitation can be used to reconstruct past palaeoclimatic temperatures. δD and δ18O is related to surface temperature at middle and high latitudes. The relationship is consistent and linear over Antarctica.
Snow falls over Antarctica and is slowly converted to ice. Stable isotopes of oxygen (Oxygen [16O, 18O] and hydrogen [D/H]) are trapped in the ice in ice cores. The stable isotopes are measured in ice through a mass spectrometer. Measuring changing concentrations of δD and δ18O through time in layers through an ice core provides a detailed record of temperature change, going back hundreds of thousands of years.
An example of using stable isotopes to reconstruct past air temperatures is a shallow ice core drilled in East Antarctica. The presence of a “Little Ice Age”, a cooler period ending ~100 to 150 years ago, is contested in Antarctica. Disparate records often provide conflicting evidence. This ice core attempted to investigate the evidence for cooler temperatures during this period.
A 180 m deep ice core from the Ross Sea, Antarctica, was drilled by a team led by Nancy Bertler in 2001/2002. The top 50 m of the ice core was analysed at 2.5 cm resolution using a continuous melting system. Ice core samples were analysed for stable isotope ratios, major ions and trace elements. An age model was extrapolated to the ice core using a firm decompaction model. Deuterium data (δD) were used to reconstruct changes in summer temperature in the McMurdo Dry Valleys over the last 900 years. The study showed that there were three distinct periods: the Medieval Warm Period (1140 to 1287 AD), the Little Ice Age (1288 to 1807 AD) and the Modern Era (1808 to 2000 AD).
These data indicate that surface temperatures were around 2°C cooler during the Little Ice Age, with colder sea surface temperatures and possibly increased sea ice extent, stronger katabatic winds and decreased snow accumulation. The area was cooler and stormier.
Past greenhouse gasses
The most important property of ice cores is that they are a direct archive of past atmospheric gasses. Air is trapped at the base of the firn layer, and when the compacted snow turns to ice, the air is trapped in bubbles. This transition normally occurs 50-100 m below the surface. The offset between the age of the air and the age of the ice is accounted for with well-understood models of firn densification and gas trapping. The air bubbles are extracted by melting, crushing or grating the ice in a vacuum.
This method provides detailed records of carbon dioxide, methane and nitrous oxide going back over 650,000 years. Ice core records globally agree on these levels, and they match instrumented measurements from the 1950s onwards, confirming their reliability. Carbon dioxide measurements from older ice in Greenland is less reliable, as meltwater layers have elevated carbon dioxide (CO2 is highly soluble in water). Older records of carbon dioxide are therefore best taken from Antarctic ice cores.
Other complexities in ice core science include thermal diffusion. Prior to becoming trapped in ice, air diffuses to the surface and back. There are two important fractionation processes: thermal diffusion and gravitational settling. Thermal diffusion occurs if the surface is warmer or colder than the bottom boundary (the close-off depth). This temperature gradient occurs from climate change, which affects the surface first. The heavier components of the air (like stable isotopes) also tend to settle down (gravitational settling).
Thermal diffusion and gravitational settling can be measured and analysed because the fractionation of air follows well understood principles and relationships between different stable isotopes (namely, nitrogen and argon).
Other major gases trapped in ice cores (O2, N2 and Ar) are also interesting. The stable isotope concentration (δ18O) in ice core records mirrors that of the ocean. Oceanic δ18O is related to global ice volume. Variations of δ18O in O2 in ice core gasses are constant globally, making it a useful chronostratigraphic marker. It’s another way to relate ice-core chronologies.
Other ice-core uses
The vertical profile of an ice core gives information on the past surface temperature at that location. In Greenland, glass shard layers from volcanic eruptions (tephra) are preserved in ice cores. The tephra ejected in each volcanic eruption has a unique geochemical signature, and large eruptions projecting tephra high into the atmosphere results in a very wide distribution of ash. These tephra layers are therefore independent maker horizons; geochemically identical tephra in two different ice cores indicate a time-synchronous event. They both relate to a single volcanic eruption. Tephra is therefore essential for correlating between ice cores, peat bogs, marine sediment cores, and anywhere else where tephra is preserved[12, 13].
Changes in sea ice concentrations can also be reconstructed from polar ice cores. Ice core records of sea salt concentration reveal patterns of sea ice extent over longer (glacial-interglacial) timescales. Methane sulphonic acid in near-coastal ice cores can be used to reconstruct changes and interannual variability in ice cores.
Mineral dust accumulates in ice cores, and changing concentrations of dust and the source (provenance) of the dust can be used to estimate changes in atmospheric circulation. The two EPICA ice cores (European Project for Ice Coring in Antarctica) contain a mineral dust flux record, showing dust emission changes from the dust source (glacial Patagonia). Changes in the dust emission is related to environmental changes in Patagonia.
- IPICS (International Partnerships in Ice Core Sciences)
- Ice cores and climate change (British Antarctic Survey)
- NSIDC: Ice cores in Antarctica and Greenland
- Ice core research at Victoria University of Wellington, New Zealand
- James Ross Island ice core
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2. Augustin, L., C. Barbante, P.R.F. Barnes, J.M. Barnola, M. Bigler, E. Castellano, O. Cattani, J. Chappellaz, D. DahlJensen, B. Delmonte, G. Dreyfus, G. Durand, S. Falourd, H. Fischer, J. Fluckiger, M.E. Hansson, P. Huybrechts, R. Jugie, S.J. Johnsen, J. Jouzel, P. Kaufmann, J. Kipfstuhl, F. Lambert, V.Y. Lipenkov, G.V.C. Littot, A. Longinelli, R. Lorrain, V. Maggi, V. Masson-Delmotte, H. Miller, R. Mulvaney, J. Oerlemans, H. Oerter, G. Orombelli, F. Parrenin, D.A. Peel, J.R. Petit, D. Raynaud, C. Ritz, U. Ruth, J. Schwander, U. Siegenthaler, R. Souchez, B. Stauffer, J.P. Steffensen, B. Stenni, T.F. Stocker, I.E. Tabacco, R. Udisti, R.S.W. van de Wal, M. van den Broeke, J. Weiss, F. Wilhelms, J.G. Winther, E.W. Wolff, M. Zucchelli, and E.C. Members, 2004. Eight glacial cycles from an Antarctic ice core. Nature, 429(6992): 623-628.
3. Johnsen, S.J., D. Dahl-Jensen, N. Gundestrup, J.P. Steffensen, H.B. Clausen, H. Miller, V. Masson-Delmotte, A.E. Sveinbjornsdottir, and J. White, 2001. Oxygen isotope and palaeotemperature records from six Greenland ice-core stations: Camp Century, Dye-3, GRIP, GISP2, Renland, and NorthGRIP. Journal of Quaternary Science, 16: 299-307.
4. Mulvaney, R., N.J. Abram, R.C.A. Hindmarsh, C. Arrowsmith, L. Fleet, J. Triest, L.C. Sime, O. Alemany, and S. Foord, 2012. Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history. Nature, 489: 141-144.
5. Lambert, F., B. Delmonte, J.-R. Petit, M. Bigler, P.R. Kaufmann, M.A. Hutterli, T.F. Stocker, U. Ruth, J.r.P. Steffensen, and V. Maggi, 2008. Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core. Nature, 452(7187): 616-619.
6. Brook, E.J., ICE CORE METHODS | Overview, in Encyclopedia of Quaternary Science, A.E. Scott, Editor. 2007, Elsevier: Oxford. 1145-1156.
7. Aciego, S., B. Bourdon, J. Schwander, H. Baur, and A. Forieri, 2011. Toward a radiometric ice clock: uranium ages of the Dome C ice core. Quaternary Science Reviews, 30(19): 2389-2397.
8. Abram, N.J., R. Mulvaney, E.W. Wolff, J. Triest, S. Kipfstuhl, L.D. Trusel, F. Vimeux, L. Fleet, and C. Arrowsmith, 2013. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geosci, advance online publication.
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How much ice is there in Antarctica? And if it were to melt, how much would global sea levels rise, and how quickly? Continue reading
In this new website, www.greenlandmelting.com, you can browse maps of the surface melt on Greenland in each year from 1979. You can also look at years with extreme melt events, such as 2010 and 2011.
What is climate?
First, it is important here to note the differences between weather and climate. Weather is local and what you can see out of your window; weather is the cold British winter snows in 2011 and our 2012 wet and rainy summer (these are to do with short term variations in winds and atmospheric pressures), or heat waves in the USA in July 2012.
Climate is much broader scale; we are looking at long term statistical patterns in weather. An individual flood or storm is not caused by climate change, but climate change may make extreme weather events (like floods) occur more frequently. “Climate” encompasses data such as temperature, humidity, pressure, wind, precipitation (snow or rain), and other meteorological measurements.
Throughout the last 2.6 million years (the “Quaternary Period”), the earth’s climate has oscillated many times, swinging between glacial and interglacial states (Figure 1). Over the last ~1 million years, we have experienced large ice ages and interglacials with a periodicity of around 100,000 years.
We are currently in an interglacial state, which began at the start of the Holocene, ~11,500 years ago. About 104 stages of these cold and temperate cycles have been recognised in deep ocean marine sediment cores (Figure 1) (Lowe and Walker, 1997). During glacials, large ice sheets developed in mid- to high-latitudes, including over Britain and North America.
Repeated climate oscillations
These large changes are driven by changes in the earth’s orbit around the sun – see The Quaternary Period (Table 1) (Jouzel et al., 2007). Glacials and interglacials can be further divided into stadials and interstadials, and within these we have smaller scale Dansgaard-Oeschger cycles, and then even smaller cycles, such as El-Nino and ENSO.
Climate data is therefore very noisy, and climate scientists must determine patterns in this data using complex statistical techniques. Throughout this time, carbon dioxide has mirrored temperature variations, which have formed a regular pattern.
Climate variations are a natural part of the earth system. It is therefore important, using instrumental records and proxies (such as ice cores, or microfossils in marine sediment cores), to compare current trends with those in the past (Bentley, 2010).
Is our climate changing?
The Earth warmed by an average of 0.6 ± 0.2°C during the twentieth century (Houghton et al., 2001). Rapid warming has been measured with global instrumental data since the 1800s (Figure 2). In 2019, the global average temperature was 0.95°C above the twentieth Century average, making it the second warmest year on record. The five warmest years have all occurred since 2015, and nine of the 10 warmest years since 2010 (Climate.Gov, accessed August 19, 2020).
The 2001 IPCC report stated that most of this warming was likely to have been due to an increase in greenhouse gas emissions. Later IPCC reports have stated this with ever increasing confidence. However, this average rate hides considerable variations in the rate and magnitude of warming.
Regional Rapid Warming
Climate change varies seasonally, on decadal timescales, and is geographically patchy (Mann et al., 2008). Three areas in particular have been subject to recent regional rapid warming (sensu Vaughan et al., 2003), with rates of warming far faster than the average noted in the IPCC. These regions are: north-western North America, the Siberian Plateau in northeast Asia, and the Antarctic Peninsula and Bellingshausen Sea. These areas warmed by more than 1.5°C between 1950 and 2000 AD (compared with a global mean of 0.5°C) (Mann et al., 2008).
More populated regions have atmospheric sulphate aerosols, that may mask warming. Urban meteorological stations may also record anomalous warming due to urban heat island effects. However, the Antarctic continent is free of these effects. Although direct meteorological observations are short (~60 years), trends in these areas are particularly important.
The Longer Term View
Although we have measured changing temperatures and carbon dioxide levels at short timescales (since the 1700s), surely this could just reflect natural variability over short timescales? In order to understand our climate, it is very important to look at the long-term view.
In many places, atmospheric temperatures are now warmer than they have been throughout the Holocene. Figure 3 shows that the temperature of the last 200 years is much higher than it was when Romans were making wine in southern England. However, what is really concerning is the rate of change. Temperatures have risen far more sharply in the last century than they have at any point in the last 2000 years.
This conclusion is reached again and again, and a paper by Mann et al. (2008) in PNAS show that the Earth’s temperature is anomalous in a long-term time context. Mann et al. used stacked records from a variety of sources to create a global graph of temperature change. This famous ‘hockey stick’ graph (Figure 4) clearly demonstrates the rapid rate of change since the industrial revolution, with temperature rise sharply accelerating over the last 150 years.
The Pages2k team have compiled global temperature records over the last 2000 years (Pages 2k Team, Nature Geoscience 12, 643-649). This reconstruction comes from many different kinds of proxy records, including tree rings, cave deposits, corals, etc.
Ed Hawkins (website: Climate Lab Book) has visualised the Pages2k climate record for the years 0 AD to 2019 AD using #warmingstripes (Figure 5). The colours highlight the recent very rapid warming.
The uncertainty estimates mean that you can also view the dataset in a more traditional way. The Medieval Warm Period and Little Ice Age are clearly shown, and the highly unusual recent rapid warming. The Medieval Warm Period and Little Ice Age are relatively small phenomena compared with the recent rapid warming.
Climate change over the last 11,300 years
A paper out in Science by Marcott et al. (2013) extends Figures 3 and 4 back to the last 11,300 years, with the Earth’s temperatures now warmer than the Earth was 4000 years ago (Figure 7). During the last 200 years, temperatures have rocketed up, confirming earlier reconstructions by Mann et al. 2008.
Although the Earth has not yet exceeded the temperatures of the Early Holocene (5000 to 11,000 years ago), global temperatures have risen from cooler than 95% of the Holocene at around 1900 to warmer than 72% of the Holocene in the last 100 years (Figure 7).
This means that, in the last 100 years, the Earth’s temperature has reversed a long-term cooling trend that began around 5000 years ago to become near the warmest temperatures during the last 11,000 years. Furthermore, climate models predict that the Earth’s temperature will exceed the warmest temperatures of the Holocene by 2100, regardless of which greenhouse gas emission scenario is used (Marcott et al., 2013).
However, Marcott et al. do note in their paper that the ‘uptick’ shown in the graph is not statistically robust, as the median resolution of all data is 120 years. However, this has been shown by other authors, such as Anderson et al. 2013, who demonstrate rapid warming over the last century from geological data.
Kaufman et al. (2020) provide an updated global temperature reconstruction, dating back to 12,000 years ago (Figure 8). They used a median from five different reconstruction methods, generating a consensus global mean surface temperature reconstruction. It follows the same approach as the PAGES 2k Consortium.
Kaufman et al. (2020) found that the warmest period in the Holocene was centred at 6,500 years ago, and was 0.6°C warmer than the 1800-1900 reference period.
The decade of 2011-2019 averaged 1 °C higher than 1850–1900 (Kaufman et al., 2020). For 80% of the ensemble members, no 200 year interval in the last 12,000 years was warmer than the last decade. Temperatures projected in the rest of this century, and beyond, are very likely to exceed 1 °C above pre-industrial temperature (IPCC, 2013).
The temperatures reconstructed vary in the different latitudes and different hemispheres; the palaeoclimate of the Northern and Southern hemispheres is very different (Figure 9).
Below, you can see the Kaufman et al. 2020 reconstruction, compared with other published proxy reconstructions over this time period. There are more proxy datasets from the Northern Hemisphere than the Southern Hemisphere.
Going back to the Last Glacial Maximum
However, it doesn’t end there. Using more proxies to extend the temperature graph back to the last glacial and modelling scenarios from the IPCC for the next 100 years, the graph below (Figure 11) has been created. Dubbed “The Wheelchair”, it shows the current and future alarming rate of temperature increase, compared with temperature fluctuations over the last 20,000 years. This figure was created by Jos Hagelaars.
The role of carbon dioxide
Carbon dioxide is a key factor in climate change. Carbon dioxide concentrations fluctuate annually, as seen in Figure 12 from the longest direct measurements of CO2 from Mauna Loa in Hawaii.
You can explore the changing global levels of carbon dioxide, methane, the global temperature record over the last 2000 years and global mean sea level here.
The seasonal fluctuation is caused by variations in uptake of carbon dioxide by land plants. However, looking at Figure 2 above, you can see that this is just part of a much longer record of carbon dioxide. The IPCC 2007 Synthesis Report stated:
“Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004”.IPCC 2007
These increases in carbon dioxide are primarily due to increased use of fossil fuels, but land use changes also provide a significant contribution. You can explore global energy production at the Global Electricity Review website.
Because humans are constantly adding more carbon dioxide to the atmosphere, we are changing its balance and affecting the Earth’s climate. The carbon dioxide and other atmospheric gasses in the atmosphere combine with changes in land cover and solar radiation to drive climate change. They affect the absorption, scattering and emission of radiation within the Earth’s atmosphere, resulting in a positive energy balance and warming influences on our climate.
The IPCC 2007 states that there is very high confidence that the world has warmed since 1750, and that the combined radiative forcing due to increases in carbon dioxide, methane and nitrous oxide is 2.3W/m2, and that the rate of increase during the industrial era is was unprecedented in the last 10,000 years.
Changes in solar irradiance since 1750 AD have caused a small radiative forcing of +0.12 W/m2.
But how has carbon dioxide changed over the last few thousand years?
Figure 14 shows the long-term temperature and carbon dioxide record from Antarctica. Carbon Dioxide (blue line) now approaches 400 parts per million, which is far higher than it has been at any point in the last 400,000 years.
Methane (green line) is also higher than it has been before over this timescale. You can also see how closely temperature (red line) has tracked carbon dioxide and methane over this time. The temperature in Antarctica (red line) is expected to continue to rise with this increase in atmospheric greenhouse gases.
Climate change in Antarctica
The western Antarctic Peninsula warmed by 2.5°C from 1950-2000 (Turner et al., 2005) (Figure 15). The mean annual temperature is rising, with the -9°C isotherm moving southwards, resulting in the collapse of several ice shelves (28,000 km (Turner et al., 2005) of ice shelf has been lost since the 1990s) (Morris and Vaughan, 2003). Climate records from the South Orkney islands suggest that warming began in earnest in the 1930s (Mann et al., 2008).
Southern Westerly Winds
This warming has been related to variations in the belts of upper atmosphere winds that encircle the Antarctic continent (the Circumpolar Vortex) (Figure 16).
Periodic oscillations in this result in the periodic strengthening and weakening of this belt of winds. A strengthening of this vortex has been associated with changes in surface atmospheric pressures (van den Broeke et al., 2004). This pressure pattern causes northerly flow anomalies, which in turn cause cooling over East Antarctica and warming over the Antarctic Peninsula.
Other factors contributing to the recent regional rapid warming over the Antarctic Peninsula include decreased sea ice in the Bellingshausen Sea, resulting in warmer air temperatures, and decreasing precipitation over the south western peninsula (van den Broeke and van Lipzig, 2004; van Lipzig et al 2004).
Finally, since the 1950s, the Antarctic Circumpolar Current has warmed by +0.2°C, with the warming greatest near the surface (Gille, 2008). Waters west of the Antarctic Peninsula have also warmed rapidly (Turner et al., 2005; Mayewski et al. 2009).
This video from NASA shows global warming since 1884, and you can really see the large and rapid warming around the Antarctic Peninsula over the last 50 years.
Observed impacts of climate change
The impacts of this recent regional rapid warming around the Antarctic Peninsula have been dramatic, with the collapse of ice shelves (Cook and Vaughan, 2010), and with 87% of glaciers in recession (Cook et al. 2005). The present-day ice loss from the Antarctic Peninsula is -41.5 giga-tonnes per year (Ivins et al. 2011). Ice-shelf tributary glaciers have become destabilised following ice-shelf collapse (Scambos et al. 2004; DeAngelis and Skvarca, 2003).
Other glaciers have thinned, accelerated and receded as a result of increased melting (see Recent Change) (Pritchard et al. 2009; Pritchard and Vaughan, 2007). The rapid shrinkage of glaciers around the Antarctic Peninsula, coupled with the potential for ice-shelf collapse and grounding line retreat, raises concerns for the future of the West Antarctic Ice Sheet, and this is an area of urgent current research (Bentley, 2010). This is covered in more detail under Marine Ice Sheet Instability.
Videos to watch
- Marine Ice Sheet instability
- Ice shelves
- Sea level rise
- Glacier recession in Patagonia
- Glacier recession on the Antarctic Peninsula
- Glaciers and climate change
- Antarctica’s contribution to global sea level rise
- NASA Global Climate Change page
- Ed Hawkins Climate Lab Book
- Warming Stripes
- Warning Stripes
- Skeptical Science
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Bentley, M.J., 2010. The Antarctic palaeo record and its role in improving predictions of future Antarctic Ice Sheet change. Journal of Quaternary Science, 2010. 25(1): p. 5-18.
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