Polar reinforcement

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A change in radiative forcing leads to a global climate change. The associated temperature changes are far more pronounced at the poles than at other places on the earth's surface. This phenomenon was called Polar Amplification by Syukuro Manabe and Ronald J. Stouffer in 1980 ; the term has since established itself. In the German-speaking world, the translation polar amplification or polar amplification is often used . In relation to the northern or southern polar region, one also speaks of Arctic amplification or Antarctic amplification .

Processes

Dark water surfaces absorb more heat than light snow and ice surfaces

Different physical processes play a role for polar amplification, depending on the season.

The ice-albedo feedback is seen as the most important process: Snow and ice surfaces reflect up to 90% of the radiated solar energy into space. The melting of the snow and ice surfaces reveals the underlying land and water surfaces, which absorb a larger part of the solar energy (hence their darker color). The absorbed energy additionally heats the surface.

However, climate simulations show that this is only the second most important effect; the polar amplification can also be observed entirely without the influence of the albedo change. The most important effects here are the changed atmospheric temperature gradient in the Arctic and the increased radiation at higher temperatures. Since the atmosphere at the poles is flatter than at low latitudes, less mass has to be heated up. Due to the Stefan-Boltzmann law , the radiated power increases with the fourth power of the temperature , measured in Kelvin . If one denotes the change in the emitted power density (= power per unit area) with and the temperature change with , the relationship follows by differentiating the Stefan-Boltzmann law .

A change in the power density of 1 W / m 2 therefore requires a temperature increase of 0.31 K at an initial temperature of -30 ° C (= 243 K), but only a rise in temperature at an initial temperature of +30 ° C (= 303 K) 0.16 K.

The decline in sea ice cover and thickness also means that the sea in these areas loses its isolation from low air layers. In summer the water stores more heat from these air layers. In autumn, when solar radiation ends, the ocean, which is relatively warm compared to the atmosphere, gives off its heat to the atmosphere. Sea water has a considerably higher heat capacity than air or rock. It therefore takes a comparatively long time for ice to form. During the onset of ice formation, latent heat is also released into the air. The processes associated with the decline in sea ice lead to increased warming of the air above the sea, particularly in autumn and winter, and subsequently to thinner ice in spring. Over land areas, on the other hand, the air near the surface warms up, especially in spring, because here only the snow or ice albedo feedback has an effect. Due to the lower temperatures in the Arctic compared to lower latitudes, warming only leads to evaporation to a lesser extent, so that more energy is available for warming the air.

In addition, warming leads to altered oceanic and atmospheric circulations, which promotes further warming. Changes in the net heat transport in the atmosphere and due to ocean currents, for example due to the Atlantic Multi-Decade Oscillation , explain an observed increased warming of higher air layers in summer.

In contrast to lower latitudes, clouds in the Arctic have a warming rather than cooling effect. They increase the atmospheric reflection and hinder the release of heat into space. In contrast, their albedo does not play a cooling role in the dark arctic autumn and winter. Cloud cover in the Arctic is particularly sensitive to changes in surface albedo. More free sea areas and higher evaporation and possibly also the increased transport of moist air from low latitudes lead to higher water vapor content and higher cloud cover in the Arctic. At the same time, these could in turn cause increased heat transport from lower latitudes. Simulations suggest that the warming caused by the cloud feedback could exceed that of the ice-albedo feedback.

Recent research suggests that the decline in the concentration of cooling sulfate aerosols and the increase in warming soot particles also play an important role in the current warming of the Arctic. The increase in melt pools on ice surfaces can also increase warming by reducing surface albedo and contributing to sea ice melt.

The change in vegetation in arctic regions decreases the albedo and increases evapotranspiration, but can also shade the soil and protect permafrost . In sum, changes in vegetation are likely to further intensify warming at high latitudes.

Observations

Increased temperature trend in the Arctic 1981–2009

Paleoclimatic studies indicate that previous temperature fluctuations in the Arctic were three to four times greater than the fluctuations in the entire northern hemisphere. Instrumental measurements now clearly show the current arctic amplification. The warming trend in the region between 70 ° N and 90 ° N in the years 1970–2008 was about three times the global warming trend. The arctic amplification is particularly pronounced over the arctic ocean and in autumn and winter. Simulations of future climate developments suggest that it will increase over the next few decades.

Differences between the Arctic and Antarctic

If climate models are calculated until a stable state of equilibrium occurs (centuries to several millennia), the polar amplification can be observed in the Arctic as well as in the Antarctic. For the past century, measured data, but also model calculations for the Antarctic - with the exception of West Antarctica - showed no observable polar amplification. The reason is that the much larger water masses and the deep ocean circulation of the South Pacific largely absorbed the heat introduced. Arctic ice is primarily sea ice. This is not only melted from above, by the sun, but also from below, by warming seawater. Most of the Antarctic ice is on land and will almost completely cover these land areas in the 21st century. A snow and ice albedo feedback as in the Arctic is therefore not to be expected in the Antarctic in the near future. For these reasons, polar amplification in the Antarctic is only recognizable after a very long time after increasing the greenhouse gas concentration in climate models. The ozone hole resulted in the 20th century in Antarctica even a slowdown in the second half.

From 1992 to 2017 an increase in the East Antarctic ice sheet was observed, but this was more than offset by a decrease in the West Antarctic ice sheet over the same period. The ice masses in the sea are larger in some regions of the Antarctic, but have become smaller in others. According to a study published in Nature in 2016, one possible explanation for the differences between the Arctic and the Antarctic is long-term intra-ocean fluctuations in the Pacific, with the tropical eastern Pacific having been cooling since 1999.

Web links

Individual evidence

  1. ↑ Syukuro Manabe , Stouffer, Ronald J .: Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere . In: Journal of Geophysical Research . 85, No. C10, January 1980, pp. 5529-5554. doi : 10.1029 / JC085iC10p05529 .
  2. ↑ Kristina Pistone, Ian Eisenman, Veerabhadran Ramanathan: Radiative Heating of an Ice-Free Arctic Ocean . In: Geophysical Research Letters . tape 0 , no. 0 , ISSN  1944-8007 , doi : 10.1029 / 2019GL082914 ( wiley.com [accessed July 16, 2019]).
  3. ↑ a b Arctic Climate Impact Assessment (2004): Arctic Climate Impact Assessment . Cambridge University Press , ISBN 0-521-61778-2 , see online
  4. ^ V. Ramanathan, A. Inamdar: Chap. 5: The radiative forcing due to clouds and water vapor . In: Frontiers in Climate Modeling (Eds. JT Kiehl, V. Ramanathan), Cambridge University Press , 2011, ISBN 978-0-521-29868-1 .
  5. ↑ Felix Pithan, Thorsten Mauritsen: Arctic amplification dominated by temperature feedbacks in contemporary climate models . In: Nature Geoscience . February 2, 2014. ISSN 1752-0894 . doi : 10.1038 / ngeo2071 .  
  6. ↑ a b c d e f Mark C. Serreze, Roger G. Barry: Processes and impacts of Arctic amplification: A research synthesis . In: en: Global and Planetary Change . tape 77 , no. 1-2 , May 2011, pp. 85-96 , doi : 10.1016 / j.gloplacha.2011.03.004 .
  7. ↑ M. Nicolaus, C. Katlein, J. Maslanik, S. Hendrick: Changes in Arctic sea ice result in Increasing light transmittance and absorption . (PDF) In: Geophysical Research Letters . 39, No. 24, December 2012. doi : 10.1029 / 2012GL053738 . (accessed on August 26, 2016).
  8. ↑ Richard G. Perso et al. a .: Shifts in Arctic vegetation and associated feedbacks under climate change . In: Nature Climate Change . March 31, 2013, doi : 10.1038 / nclimate1858 .
  9. ↑ David WJ Thompson et al. a .: Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change . In: Nature . 2011, doi : 10.1038 / NGEO1296 (Review Article).
  10. ↑ Andrew Shepherd et al. (The IMBIE team): Mass balance of the Antarctic Ice Sheet from 1992 to 2017 . (PDF) In: Nature . 556, June 2018, pp. 219–222. doi : 10.1038 / s41586-018-0179-y .
  11. ^ John King: A resolution of the Antarctic paradox . In: Nature . tape 505 , January 23, 2014, p. 491-492 ( nature.com ).
  12. ↑ Gerald A. Meehl, Julie M. Arblaster, Cecilia M. Bitz, Christine TY Chung, Haiyan Teng: Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability . In: Nature Geoscience . tape 9 , 2016, p. 590-595 ( nature.com ).