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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Ocean Controls on high-latitude Climate sensitivity - a Pliocene case study

Awarded: NOK 8.8 mill.

Project Number:

221712

Application Type:

Project Period:

2013 - 2017

Location:

Partner countries:

Warm climate - at present and in the past We live in a time when man-made climate change is on of the most important topics society has to deal with. Among scientists working on climate related topics there is an overall agreement that humans change climate beyond natural climate variability. These changes manifest themselves in higher global temperatures, with enhanced warming at high latitudes, a reduced sea ice cover in the Arctic Ocean and higher sea level. It is also an overall agreement between scientists that Earth have experienced warm climate earlier. However, the fact that it has been warm before does not mean that we should not be worried about the on-going climate changes. There are many reasons why past warm climate are not directly comparable to the present warming. One difference is that the increase in atmospheric CO2 that have been taking place since the beginning of the industrial revolution occurs faster than any change known from geological times. When studying past warm climate, we gain information on long-term effects of and a climate in equilibrium with a high atmospheric CO2 level. The last time Earth experienced atmospheric CO2 values comparable with those measured today were in the Pliocene epoch (5.3-2.6 million years ago). Studying this time period helps us understand how our local climate may be during a warm climate. In the project "OCCP: Ocean Controls on high-latitude Climate sensitivity - a Pliocene case study", we have investigated how and why climate changed as it did in the Nordic Seas. We have reconstructed the climatic and oceanographic conditions based on studies of marine sediment cores, and we have used the Norwegian Earth System Model to investigate the effect of changing the radiative forcing and the effects of known geographical differences between the Pliocene and the present. Climate was not stable through the two million yearlong time interval investigated. Both larger and smaller, and shorter and longer lasting, climatic changes took place. The larger changes that occurred represents shifts every 400,000 to 200,000 years, between locally warmer and colder climate regimes. Overall, it was warmer than today, both in the ocean and in the atmosphere, in line with the overall higher atmospheric CO2 concentrations. However, during some of these climate states temperature of both the ocean and the atmosphere were comparable to todays. The transitions between these warmer and colder climate states occurred as a response to geographical changes, in combination with responses to low frequent orbital insolation forcing. Examples of geographical differences that influenced the Nordic Seas climate was that the Greenland-Scotland ridge was deeper than today, and that what is presently the Barents Sea was not a sea but a land area. The deeper Greenland-Scotland ridge entailed warmer sea surface temperatures in the Nordic Seas, while an exposed Barents Sea would in itself cause a slight cooling in the Nordic Seas. A change in the ocean current regime through the Bering Strait was another important factor that influenced the climate development at high northern latitudes and how the temperature gradients in the Nordic Seas developed. The Pliocene temperature development in the Norwegian Sea is closely connected to the temperature development of the North Atlantic, however, it was not always the same. The temperature development in the Norwegian Sea and over Norway was however very similar. When sea surface temperatures in the Norwegian Sea were comparable to todays, the vegetation in Northern Norway was also comparable to todays. However, when sea surface temperatures in the Norwegian Sea were warmer by 3-4°C, the Northern Norwegian vegetation was similar to the vegetation presently found in southern Norway.

The Pliocene Epoch (5.3-2.6 million years ago) is a warm period of particular interest, when boundary conditions of the climate system were similar to the present, but sea level was higher and today`s polar and subpolar regions were as warm as or warmer t han predicted in future scenarios. High northern latitudes are thought to have been especially warm; however, how these warm, high-latitude conditions were maintained is an open question, in part given the apparently contradictory observational evidence i n the Nordic Seas. The Nordic Seas link the North Atlantic and Arctic Oceans, and is a small but dynamically important part of the global ocean. In today`s climate, they form a region of strong atmosphere-ocean exchange, vigorous mixing of water masses a nd deep convection. The Nordic Seas offer a unique opportunity to study the exchange of salt, heat and mass between the North Atlantic and Arctic oceans. OCCP aims to resolve conflicting views on the role of the Nordic Seas in determining high northern la titude climate during a key warm period in Earth`s history, the Pliocene. We will adopt a multidisciplinary approach. New, multi-proxy data characterizing the Nordic Seas water column during the Pliocene will be generated and combined with published data from the North Atlantic/Arctic region. The reconstructions will be combined with results from model experiments designed to investigate the large-scale dynamics of the region. The model results will be used to identify key processes responsible for the ob served high latitude climate of the Pliocene and the role of the Nordic Seas "gateway" between the North Atlantic and Arctic. This approach will enable us to make substantial progress towards understanding the mechanisms behind the apparent high sensitivi ty of the Arctic in warm climates, the role of the Nordic Seas in the arctic amplification of Pliocene warming, and will ultimately help constrain the long-term sensitivity of the climate system in a warming world.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek