Climate: processes and drivers

What happens in the global climate is mainly determined by a few fundamental processes: incoming solar radiation, characteristics of the earth’s surface, the atmosphere’s ability  to retain heat, and the reflectivity of the atmosphere and the earth’s surface. Various mechanisms serve to enhance or weaken the effects of these processes on climate.

Global processes

Albedo effect at sea

Albedo effect at sea. A light surface (snow and ice) reflects almost 80% of the incoming energy back to the atmosphere, whereas the dark ocean absorbs heat and reflects only about 10%.
Illustration: Audun Igesund / Norwegian Polar Institute

The energy that radiates from the sun creates the basis for weather and climate on earth. The radiation absorbed makes the earth warmer. Unless an equal amount of energy is lost to outer space, the temperature on earth would increase. Earth loses energy to space by radiating infrared light from the surface and the atmosphere. Averaged over the entire globe, the earth loses the same amount of energy in the form of infrared radiation as it takes up from the sun.

For celestial bodies without an atmosphere, such as the moon or Mercury, it is easy to calculate surface temperature based simply on their distance from the sun, their size and how much sunlight they reflect. If the same formula is applied to the earth, the calculated average surface temperature is about ‑17°C. However, the gases in the atmosphere take up much of the infrared radiation emitted from the surface of the earth, which means that the atmosphere grows warmer. The warm atmosphere subsequently emits infrared radiation both out toward space and back to the surface. The infrared light emitted down toward earth warms the surface. This process is called the greenhouse effect, and explains why the earth has an average temperature closer to +14°C than to ‑17°C. Water vapour is the most important greenhouse gas, followed by carbon dioxide (CO2) and methane (CH4).

Climate can change as a result of natural processes or human activities. The most important process behind the ongoing climate change is an increased concentration of CO2 and other greenhouse gases in the atmosphere, which enhance the greenhouse effect. The latest IPCC report summarising available knowledge and evidence shows that the concentration of CO2 in the atmosphere has increased by about 40% since the beginning of the industrial revolution. There are clear indications that human activities have caused this increase. The current atmospheric concentration of CO2 is far higher than any level attained through natural variation over the past 800 000 years, as demonstrated by ice cores, and it is quite certain that the increase in atmospheric CO2 levels seen in the last 100 years has been more rapid than any other increase over the last 22 000 years.[1] Read more about the greenhouse effect and changing concentrations of greenhouse gases at miljøstatus.no.

Aerosols (tiny particles of soot or sulphates) in the atmosphere can have a cooling effect owing to their ability to refract and absorb incoming solar radiation. The aerosols can also have an indirect effect: they function as condensation nuclei and contribute to formation of clouds. Increased cloud cover increases the earth’s ability to reflect sunlight and thus cools the earth. However, soot in aerosol form also has a warming effect. Read more about soot  as a driver of climate at miljøstatus.no. Human activities release many aerosols. The IPCC estimates that overall, man-made aerosols have a cooling effect; in other words, aerosols have lessened the warming we would otherwise have experienced from the increased concentrations of greenhouse gases.[1]

Many other natural processes also influence climate. These processes have led to major climate changes in the past. In the past few million years, the earth has experienced several ice ages, when ice sheets like those that now cover Greenland and Antarctica covered large parts of North America and Europe. These changes were mainly caused by gradual changes in the earth’s orbit around the sun.

Solar radiation varies over an 11-year cycle, and also over longer time scales. The latest IPCC summary of available knowledge and evidence shows that changes in solar radiation have probably contributed very little to the overall changes in climate since the beginning of the industrial era.[1] A few studies show that changes in solar radiation may have contributed to increased global average temperatures during the first half of the 20th century, but have probably played a very minor role in the last half of the century.[3]

The global climate system is also regulated by the energy balance in the oceans and the atmosphere. Global ocean circulation and atmospheric circulation are driven by forces that strive to even out differences in temperature between high and low latitudes. Heat exchange between ocean and atmosphere is an important factor in regional climate patterns. Conditions that influence this balance – such as changes in air and sea temperatures, or cloud and sea ice cover – will thus influence how the climate evolves. On a geological time scale, changes in the shape and location of continents can have strong effects on circulation and heat balance and thus also on global climate. However, given that the continents have been in approximately the same place for the past 500 000 years, this is not a factor of any importance for ongoing climate change.

On geological time scales, the concentrations of greenhouse gases – especially CO2 – change through natural processes. Volcanos emit CO2 to the atmosphere. This release is balanced by processes that capture CO2 in the seabed, and it can be demonstrated in several ways that the increase in atmospheric CO2 levels since the industrial revolution has been caused by human activities.

Solar radiation varies over an 11-year cycle, and also over longer time scales. The latest IPCC summary of available knowledge and evidence shows that changes in solar radiation have probably contributed very little to the overall changes in climate since the beginning of the industrial era.[1] A few studies show that changes in solar radiation may have contributed to increased global average temperatures during the first half of the 20th century, but have probably played a very minor role in the last half of the century.[3]

The global climate system is also regulated by the energy balance in the oceans and the atmosphere. Global ocean circulation and atmospheric circulation are driven by forces that strive to even out differences in temperature between high and low latitudes. Heat exchange between ocean and atmosphere is an important factor in regional climate patterns. Conditions that influence this balance – such as changes in air and sea temperatures, or cloud and sea ice cover – will thus influence how the climate evolves. On a geological time scale, changes in the shape and location of continents can have strong effects on circulation and heat balance and thus also on global climate. However, given that the continents have been in approximately the same place for the past 500 000 years, this is not a factor of any importance for ongoing climate change.

On geological time scales, the concentrations of greenhouse gases – especially CO2 – change through natural processes. Volcanos emit CO2 to the atmosphere. This release is balanced by processes that capture CO2 in the seabed, and it can be demonstrated in several ways that the increase in atmospheric CO2 levels since the industrial revolution has been caused by human activities.

Processes at the poles

Sea ice is an important factor in the global climate system

Sea ice is an important factor in the global climate system. Photo: Odd Harald Selboskar / Norwegian Polar Institute

Distinctly polar processes in both north and south, on land (snow, glaciers, and permafrost) and at sea (sea ice, ocean circulation, bottom water formation) ), play a crucial role in the global climate system, acting through complex interactions and  feedback mechanisms.

Sea ice is an important factor in maintaining radiative balance in the global climate system through the  albedoeffekten. . Snow-covered sea ice reflects about 80% of incoming solar radiation, in contrast to open seas, which absorb more than 90% of incoming solar radiation and reflect only 10% back to the atmosphere. Because of this, changes in the proportion of sea ice and open water have a strong impact on the climate in this region. Record low amounts of sea ice are now being observed repeatedly in Arctic, whereas the extent of sea ice around Antarctica is relatively stable or increasing slightly. Studies suggest that the changes in ice cover in the north over the past decades have contributed to warmer temperatures in the Arctic through much of the year. They also suggest that most of the recent temperature increase in the Arctic can be attributed to reduced sea ice coverage, which in turn influences the formation of sea ice.[6][7] A study from 2010 concluded that the changes in Arctic sea ice extent in the past few years have had less impact on temperature trends outside the region, that is south of 60°N.[8]

Altered ice dynamics and structure, combined with uptake of heat in ice-free seas help enhance the warming of the Arctic and the loss of sea ice. When the heat stored in this reservoir returns to the atmosphere in the autumn and winter, the warmth does not stay in the lower layers of the atmosphere, but rises to higher altitudes, where it influences Arctic wind systems, particularly air exchange between north and south.[9] This is probably a contributing factor in the record low temperatures and record heavy snowfall in southern Europe, along with unusually high temperatures in the Arctic in the winter of 2009-2010.

At the poles, cold, dense water is formed, which flows along the depths of the oceans toward the equator; to compensate, other currents form and flow at the ocean surface toward the poles. This is the motor in the ocean circulation system, which in turn regulates global climate. New bottom water forms in only a few areas of the world’s oceans. Global warming can perturb bottom water formation by warming the surface water and increasing the influx of fresh water, both of which decrease the density of the surface water. A considerable proportion of the fresh water in the Arctic Ocean leaves the Arctic with the East Greenland Current through Fram Strait and ends up in the Greenland and Labrador seas, where it can influence the crucial bottom water formation. The Norwegian Polar Institute has been  monitoring the fresh water current in Fram Strait since 1997, through permanently deployed instruments and annual research cruises across the current. The Institute reports these monitoring results in  MOSJ.

Snow cover, like sea ice cover, is an important factor in maintenance of radiation balance in the global climate system through the albedo effect. On average, about 46 million square kilometres of the earth’s surface is covered with snow every year. But the total area of this snow cover is decreasing, and the period when there is snow cover is getting shorter. The latest IPPC report[1] shows that over the last decades, the area covered by snow has decreased by about 1.6% per decade, and the spring snowmelt is occurring earlier and earlier. Studies imply that the changes in surface temperature that result from changes in snow cover are smaller than those caused by altered sea ice coverage, but are more extensive and prominent in autumn and spring.[10]

Permafrost lies under much of the land in the Arctic, and under the seabed in some places. Permafrost is important for global climate developments because huge amounts of greenhouse gases (mainly methane) lie “locked” inside the frozen ground and could be set free if the permafrost were to disappear. Permafrost is thawing at several locations in the Arctic, and its temperature is now 2°C warmer than it was 20-30 years ago.[11] A monitoring series reported through MOSJ, shows thawing also in Svalbard. So far, however, it has been difficult to calculate the potential magnitude of greenhouse gas emissions from thawing permafrost, because many of the interlinked consequences of such thawing remain poorly understood. The most recent IPCC summary of available knowledge and evidence shows that the best estimate for 2100 is between 50 and 250 gigatonnes of carbon, depending on how global temperature evolves.[1]

Glaciers and ice sheets in polar regions influence the climate system in several ways. They too affect radiation balance through the  albedo effect , just as sea ice and snow do, but they also have impact on influx of fresh water to the world’s oceans, and thus affect ocean circulation. Almost all the glaciers and ice caps in the Arctic have decreased in volume over the last century; Alaska and northern Canada are the regions that have seen the greatest loss of glacier mass in the past decade.[1] Parallel with this, reduced seawater salinity and density have been observed. It has been estimated that the influx of fresh water (from all sources) has increased by 7700 km3 over the past few years. If this trend continues, there is a risk of changes in major ocean currents, which would in turn have impact on global climate.[11] 

References

  1. Intergovernmental Panel on Climate Change (IPCC) 2013. Fifth assessment report contribution.
  2. Working Group I contribution to the IPCC 5th Assessment Report "Climate Change 2013: The Physical Science Basis"
  3. L. J. Gray et al. 2010. Solar influences on climate. Reviews of Geophysics (48): 4. DOI:10.1029/2009RG000282
  4. Stephen R. Hudson et al. 2013. Energy budget of first-year Arctic sea ice in advanced stages of melt. Geophysical Research Letters 40 (11).2679–2683 DOI:10.1002/grl.50517
  5. Stephen R. Hudson.2011. Estimating the global radiative impact of the sea ice–albedo feedback in the Arctic.Journal of Geophysical Research: Atmospheres (116) D16, 27. DOI:10.1029/2011JD015804
  6. James Screen & Ian Simmons 2010. The central role of diminishing sea ice in recent Arctic temperature amplification. Nature 464, 1334–1337. DOI:10.1038/nature09051
  7. Julienne C. Stroeve et al. 2012. Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations. Geophysical Research Letters. (39) 16 : 6983–6992. DOI:10.1029/2012GL052676
  8. Arun Kumar et al. 2010. Contribution of sea ice loss to Arctic amplification. Geophysical Research Letters (Volume 37, Issue 21). DOI:10.1029/2010GL045022
  9. James E. Overland et al. 2011. Warm Arctic—cold continents: climate impacts of the newly open Arctic Sea. Polar Research (30): 15787. DOI:10.3402/polar.v30i0.15787
  10. Michael A. Alexander et al. 2010. The Atmospheric Response to Projected Terrestrial Snow Changes in the Late Twenty-First Century. J. Climate, 23, 6430–6437.. DOI:10.1175/2010JCLI3899.1
  11. Arctic Monitoring and Assessment Programme (AMAP), 2012. Arctic Climate Issues 2011: Changes in Arctic Snow, Water, Ice and Permafrost. Snow, Water, Ice and Permafrost in the Arctic (SWIPA) 2011 Overview Report.