GeoLog

climate change

Geosciences column: Playing back the Antarctic ice records

Satellites are keeping tabs on the state of Arctic and Antarctic sea ice, and have observed considerable declines in ice extent in many areas since records began, but what do we know of past sea ice extent?

Ice cores keep an excellent record of climate change, but until recently, ice cores have not been used to quantify patterns in past sea ice extent because few reliable compounds are preserved in the ice. While methanesulphonic acid (MSA) has been used in the past, it is an unstable compound and is easily remobilised after it has been deposited. The amount of sea-salt sodium deposited in brine pools or as high salinity crystals known as ‘frost flowers’ that form on the ice surface can also be used to identify changes in sea ice extent. These deposits are difficult to distinguish from larger sources of sea-salt sodium (aerosols and sea spray) though, making it a poor proxy.

Recent research published in Atmospheric Chemistry and Physics may provide an answer. An international team of geochemists have identified two stable indicators of sea ice extent in ice cores: the halogens bromine and iodine.

The oceans are considered to the be the main reservoir of bromine and iodine, but satellite data show these halogens also have a strong link with sea ice at the poles. Furthermore, recent research suggests that algae that grow under sea ice are big contributors to atmospheric iodine. Sea ice provides a substrate for algae to grow on during the spring. It is thin enough for light to penetrate, allowing the algae to photosynthesise, and also allows compounds produced by the algae to reach the atmosphere, including iodine and iodine oxide. These peaks in iodine oxide production are spotted by satellites during the spring.

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

During interglacials, the extent of sea ice is much smaller than during a glacial period. This means that the thin sea ice (where iodine is released into the atmosphere) is much closer to the coast. Air bubbles trapped in compacting continental snow preserve these atmospheric gases, which can be sampled in an ice core at a much much later date. Thus, when there is more iodine in the ice core, the extent of sea ice is small; when there is less, the thin edge of the ice sheet was much further from the coast.

Satellite measurements also show the amount of bromine oxide in the atmosphere peaks during the polar spring. This is because the increase in light level stimulates a series of photochemical reactions that convert bromine salts into bromine and bromine oxide, releasing it into the atmosphere. These events are known as bromine explosions and result in an increase in atmospheric bromine.

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively. (Credit: Spolaor et al., 2013)

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively (click for larger). (Credit: Spolaor et al., 2013)

Because the extent of sea ice is reduced during interglacials, bromine explosions must occur closer to the Antarctic coast, meaning more bromine will be trapped in the ice sheet during an interglacial period. Using sodium as a proxy for the amount of sea salt in the ice core, Andrea Spolaor and her team were able to work out how much of the bromine was in the atmosphere at the time. Since bromine explosions result in an an increase in atmospheric bromine, but have no effect on sodium, these events can be identified when the ratio of bromine to sodium in the ice core is high.

Now we can work out the extent of past sea ice, what lies ahead? The first part of the IPCC 5th Assessment Report released earlier today concerns the physical changes in the Earth’s climate and what we can expect in the future; the future of our changing planet will be discussed at this year’s Geosciences Information For Teachers workshop at EGU 2014, and you will be able to find a great discussion of climate change, its history and its impacts in the next issue of GeoQ – stay tuned!

By Sara Mynott, EGU Communications Officer

Reference:

Spolaor, A., Vallelonga, P., Plane, J. M. C., Kehrwald, N., Gabrieli, J., Varin, C., Turetta, C., Cozzi, G., Kumar, R., Boutron, C., and Barbante, C.: Halogen species record Antarctic sea ice extent over glacial–interglacial periods, Atmos. Chem. Phys., 13, 6623-6635, 2013.

Geosciences Column: How curbing HFC emissions could reduce warming

Carbon dioxide is without a doubt the most famous of warming culprits. But would reducing emissions of this greenhouse gas be enough to mitigate climate change within this century? A recent paper published in Atmospheric Chemistry and Physics focuses on a less known substance that, if phased out, could avoid as much as 0.5 °C of warming by 2100.

Hydroflurocarbons (HFCs) have an interesting history. Now used widely as refrigerants, propellants, in fire extinguishers and air conditioning, among others, these man-made chemicals became commercially available only some two decades ago. With the adoption of the Montreal Protocol in the late 80s, ozone-depleting substances such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have been phased out and replaced by ozone-friendly chemicals such as HFCs. But while they don’t damage the ozone layer, HFCs – like their predecessors – have a very high global warming potential: 100-3000 times that of carbon dioxide.

Ozone hole over Antarctica in September 2006, the largest ever recorded. Even though the atmospheric concentrations of CFCs and HCFCs have been decreasing since around 1995 – being replaced by HFCs – their effects are still being felt given the relatively long lifetimes of these ozone-depleting substances. The ozone layer will eventually recover, but the increasing concentrations of HFCs in the atmosphere could exacerbate global warming. (Credit: NASA)

Ozone hole over Antarctica in September 2006, the largest ever recorded. Even though the atmospheric concentrations of CFCs and HCFCs have been decreasing since around 1995 – being replaced by HFCs – their effects are still being felt given the relatively long lifetimes of these ozone-depleting substances. The ozone layer will eventually recover, but the increasing concentrations of HFCs in the atmosphere could exacerbate global warming. (Credit: NASA)

The atmospheric concentration of HFCs is, however, low, accounting for only about 2% of all greenhouse gases. But as nations continue to reduce the use of ozone-depleting substances and with sales of air conditioning and refrigeration equipment booming, particularly in warm, developing countries, the use of HFCs is set to surge. “HFCs are the fastest growing greenhouse gases in the US, where emissions grew nearly 9% between 2009 and 2010 compared to 3.6% for CO2. Globally, HFC emissions are growing 10 to 15% per year and are expected to double by 2020,” the authors write in the new paper citing reports from the US Environmental Protection Agency and the World Meteorological Organization.

This potentially large increase in the use of HFCs motivated the researchers from the US and the Netherlands to find out what role HFCs play in mitigating 21st century climate change. Their aim was to determine how much warming could be avoided by replacing HFCs with readily available substitutes with a lower global warming potential (such as HFOs: hydrofluoro-olefins).

“Our calculations show that controlling HFC growth can avoid a significant amount of warming in this century, at least comparable to CO2 mitigation at 2050, and almost 50 % of CO2 mitigation by 2100,” said lead-author Yangyang Xu of the Scripps Institution of Oceanography in a press release.

Using an integrated carbon and radiant energy balance model, the researchers calculated the change in HFC radiative forcing from 2005 to 2100 under various emission scenarios. They also determined the corresponding temperature changes assuming a climate sensitivity of 0.8 °C/(Wm-2).

Model simulated temperature change under various mitigation scenarios that include CO2 and short-lived climate pollutants (BC: black carbon, CH4: methane, HFCs). The business-as-usual case (BAU, red solid line with spread) considers both high and low estimates of future HFC growths. The red-dash and black lines show the cases of CO2 mitigation and full mitigation, respectively, assuming a climate sensitivity of 0.8 °C/(Wm-2). (Image and caption adapted from Xu et al. 2013)

Model simulated temperature change under various mitigation scenarios that include CO2 and short-lived climate pollutants (BC: black carbon, CH4: methane, HFCs). The business-as-usual case (BAU, red solid line with spread) considers both high and low estimates of future HFC growths. The red-dash and black lines show the cases of CO2 mitigation and full mitigation, respectively, assuming a climate sensitivity of 0.8 °C/(Wm-2). (Image and caption adapted from Xu et al. 2013)

Their results (above) show that if only CO2 emissions are curbed, the expected warming by the end of this century is about 3.3 °C. Adding a reduction in black carbon and methane emissions, as well as HFC mitigation, lowers this value by about 1.5 °C. HFC replacement alone accounts for a drop of 0.5 °C, showing that only full mitigation can keep the 2100 temperature below 2 °C.

The study confirms the importance both the US and China are placing on cutting HFCs in their climate plans. The two countries recently agreed on phasing out production and consumption of these gases and replacing them by HFOs.

“The findings of our study provide even greater justification for phasing-down HFCs under the Montreal Protocol. It’s the biggest, fastest and cheapest climate mitigation available to the world today,” said co-author Durwood Zaelke of the Institute for Governance & Sustainable Development in a press release.

While the paper emphasises how significant HFC mitigation is, the authors point out that this should not be seen as an alternative to reducing CO2 emissions. HFCs, black carbon and methane are short-lived climate pollutants that remain in the atmosphere for up to a few decades only; CO2, on the other hand, can remain in the atmosphere for centuries to millennia. Therefore, the authors conclude, “[f]or the longer-term (century and beyond), mitigation of CO2 would be essential for a significant reduction in the warming.”

By Bárbara Ferreira, EGU Media and Communications Manager

Reference:

Xu, Y., Zaelke, D., Velders, G. J. M., and Ramanathan, V.: The role of HFCs in mitigating 21st century climate change, Atmos. Chem. Phys., 13, 6083–6089, 2013

Geosciences Column: Rainfall and Climate – a Dynamic Problem

“Rain is grace; rain is the sky descending to the earth; without rain, there would be no life.” – John Updike

Rain quenches the thirst of soils and vegetation, fuelling ecosystems and much of the world’s agriculture. Whether it ruins a day on the beach or destroys a season’s harvest, it makes humans deeply aware of their vulnerability to the vagaries of the atmosphere. It’s important to understand how rainfall changes in a changing climate. Here, I will describe the issues in understanding precipitation changes and how two recent papers help to solve the puzzle.

Predicting rainfall is difficult. It is a small-scale phenomenon, especially in the towers of convective cloud in the Tropics. Weather forecasting models are just beginning to capture them properly at scales of a kilometre or so, but climate models, which have to be run for decades rather than days, calculate atmospheric conditions on scales of hundreds of kilometres. Rainfall has to be simplified in these models, since we cannot calculate the physical properties of individual clouds. These simplified representations are called parameterisations. A precipitation parameterisation relates the average rainfall over a large area to the average amount of water in the air. Different models do this in different ways and, because it’s a simplification, there is no definitive ‘right’ way. This means there is some disagreement among climate models about how rainfall will change in the future, especially in the Tropics (areas on the figure which are not stippled).

Climate model projections of precipitation change in a future with high greenhouse gas emissions. Left: current generation of models, Right: previous generation of models (around 2005). Top: December-February, Bottom: June-August. Stippling shows areas where models largely agree. White areas show complete disagreement among models (source: Knutti & Sedlacek, 2013).

If we think about precipitation in general theoretical terms, we can find laws which must be followed and use them to make predictions, as Issac Held & Brian Soden did in their study of how the hydrological cycle responds to global warming. Rain is caused by the upward transport of water vapour from the surface into the atmosphere, where it condenses, forms clouds and rains out. The amount of moisture going up must, of course, balance the amount coming back down as rain.

As the climate warms, the amount of water vapour a fixed mass of air can hold increases. This means that, as long as the circulations transporting water upwards remain the same, the total amount of water vapour going upwards must increase – which means the amount of rain coming down must also increase. This is called the ‘rich get richer’ mechanism, because it increases rainfall in regions where there is already a lot of rain driven by upward moisture transport. It’s a fundamental mechanism driven by thermodynamic laws…but that doesn’t mean it’s the only thing going on.

Convective raincloud in tropical Africa (photo credit: Jeff Attaway).

If climate model projections followed the ‘rich get richer’ mechanism, precipitation would increase most in the regions with the most precipitation currently. In fact it is more complicated than that. Robin Chadwick and his colleagues explored the effect of weaker vertical motions in a warmer climate. We can understand this by thinking about what carbon dioxide does to the vertical temperature profile. It warms the mid-troposphere (about 5 km up) more than the surface. To get convective upward motion, the air at the surface must be less dense (i.e. warmer) than the air above. Warming the air aloft suppresses this motion. The Chadwick decomposition calculates the part of the precipitation changes caused by changes in moisture (which goes at about 7% per K) and the part caused by the reduction in upward transport. They find the two tend to roughly cancel each other out, which means the spatial shifts in precipitation are determined by changing patterns of surface temperature (since warm surfaces produce upward motion).

Sandrine Bony and her team decompose precipitation changes into two main components rather than three: one is the ‘dynamical’ component, associated with changing upward motions, and the other is the ‘thermodynamical’ component, including changes in atmospheric moisture content. Unlike the Chadwick method, the thermodynamical component is not designed solely to represent the ‘rich get richer’ mechanism. This means the thermodynamical component isn’t just a 7% per K increase; it includes things like the spatial changes in surface temperature. The dynamical component isolates the change in precipitation caused by changes in upward motion.

Monsoon raincloud over a lake in the Tibetan Plateau (photo credit: Janneke Ijmker).

The ‘rich get richer’ rule of thumb becomes increasingly irrelevant at smaller scales. This is frustrating, because these are the scales we really care about! It’s not particularly useful knowing what will happen in a general sense over the whole Tropical region. Farmers want to know what will happen to the seasonal rains on their small piece of land.

Bony also points out that geoengineering schemes which aim to reduce incoming solar radiation to cool the planet’s surface would leave the dynamical component of precipitation change untouched. This is because the dynamical component is caused by the warming of the mid-troposphere by carbon dioxide, and this remains even if we cool the surface. It is an example of the inexact nature of the cancellation between carbon dioxide increases and geoengineering schemes to decrease the amount of carbon dioxide in the atmosphere, and demonstrates that the only way to stop carbon dioxide-driven climate change properly is to stop emitting carbon dioxide.

Bony and Chadwick’s decompositions show how one can glean a lot more information from climate model projections than one would expect from first glance. We have established some general facts about climate change related to the Earth’s energy budget. In that sense we understand quite well what will happen in a warming climate. However, there is still a lot of diversity between model projections, most of which comes from differences in the dynamical response. Local changes in rainfall are related to changes in circulation, and this is the area in which a lot more work needs to be done.

By Angus Ferraro, PhD student at Reading University

References:

Bony, Sandrine, Gilles Bellon, Daniel Klocke, Steven Sherwood, Solange Fermepin & Sébastien Denvil, 2013: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation, Nat. Geosci., doi:10.1038/ngeo1799

Chadwick, Robin, Ian Boutle & Gill Martin, 2013: Spatial Patterns of Precipitation Change in CMIP5: Why the Rich don’t get Richer in the Tropics. J. Climate, doi: 10.1175/JCLI-D-12-00543.1

Held, Isaac M., Brian J. Soden, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate, 19, 5686–5699. doi: 10.1175/JCLI3990.1

Knutti, Reto & Jan Sedláček, 2013: Robustness and uncertainties in the new CMIP5 climate model projections, Nat. Clim. Change, doi: 10.1038/nclimate1716

Sussing out sea level rise

Ocean thermal expansion, that is, the increase in water volume due to temperature alone, is relatively well understood – as is the retreat of both mountain glaciers and ice caps. While most models simulate these effectively, there is little understanding of how both the Greenland and Antarctic ice sheets will respond to climate change. This is because the full extent of ice-ocean interactions is not included in climate models – it’s no wonder when there are so many factors to consider: melting of mountain glaciers, ice caps and polar ice sheets, glacial isostatic adjustment and ocean thermal expansion to name just a few!

Mahé Perrette and his colleagues have developed a novel approach for sussing out sea level rise (SLR); combining simple models with general circulation models (GCMs) to use the benefits of both in predicting future change. To get an idea of the uncertainties associated with SLR, take a look at this graph, in which red is a highly uncertain prediction (up to 70 cm variation) and dark blue is an estimate we can be quite confident of (only 5 cm variation):

The first of the three panels combines the uncertainty for all components of SLR considered by Dr Perrette and his team – there’s really not much blue there is there?! [Source: Perrette et al., 2013].

Steric changes are those that affect ocean density and dynamics through variations in temperature and salinity. Accordingly, a rise in sea level due to temperature and salinity changes is known as steric sea level rise. The magnitude of the impact of steric SLR varies widely across the globe. One of the reasons behind this is the existence of local gravity fields. The loss of ice mass due to melting means there is a lower gravitational force exerted on surrounding water masses. These changes in local gravity fields cause water to migrate away from melting ice caps.

Both the magnitude of SLR and the timescales over which it occurs varies wildly between models, but it remains clear that the effects of SLR will be felt most in the low-lying coastal regions of the world: from small island nations such as Tuvalu, which at only marginally above sea level, is a country only too aware of this threat; to the densely populated, and agriculturally important, coastal plains of Bangladesh.

Each of the contributions to SLR (meltwater from the Antarctic and Greenland Ice Sheets, and melting of mountain glaciers and icecaps) vary locally, which means that SLR will also vary from region to region. This interactive map by Perrette and his team is a great demonstration of how SLR varies throughout the world under different emissions scenarios.

Predicted sea level rise for coastal regions. The key indicates the different components contributing to SLR: MGIC = Mountain Glaciers and Ice Caps, AIS = Antarctic Ice Sheet, GIS = Greenland Ice Sheet and GIA = Glacial Isostatic Adjustment [Source: Perrette et al., 2013].

Sea level is expected to rise 25 cm higher in the Bay of Bengal than along the Dutch coast – in fact, SLR here is 10-20% higher than the global average! Why? It’s all to do with land ice and local gravity fields. Local gravitational effects suppress the effect of meltwater from the Greenland Ice Sheet along the Dutch coast, but there is no similar force keeping water back in the Bay of Bengal. In addition, land ice is responsible for a far greater portion of the SLR in the Bay of Bengal than it is along the Dutch Coast. This is no surprise, when you consider its close proximity to the Himalayas and other Asian mountains, but it adds an extra dose of uncertainty when predicting SLR here as land ice contributions to SLR remain a challenge to climate modellers.

Rates of relative sea-level change [Souce: Riva et al., 2010].

Gravitational forces influence global, as well as local, water distribution. The declining mass of ice caps at the poles means there will be weaker gravitational forces acting on Arctic and Antarctic waters. The weakening of these poleward-pulling forces will cause water to be redistributed throughout the globe, resulting in greater SLR at the equator than at the poles. So, despite the large local variations in SLR, its effects will be felt most in the tropics and at the equator.

By Sara Mynott

References:

Perrette, M., Landerer, F., Riva, R., Frieler, K., and Meinshausen, M. (2013), A scaling approach to project regional sea level rise and its uncertainties, Earth Syst. Dynam., 4, 11-29, doi:10.5194/esd-4-11-2013.

Riva, R. E. M., J. L. Bamber, D. A. Lavallée, and B. Wouters (2010), Sea-level fingerprint of continental water and ice mass change from GRACE, Geophys. Res. Lett., 37, L19605, doi:10.1029/2010GL044770.