Warmer, wetter and wilder

Prof. Ola M. Johannessen, Mohn-Svedrup Center/Nansen Environmental and Remote Sensing Center/University of Bergen

The increasing anthropogenic release of greenhouse-gases leads to warmer, wetter and wilder weather in Northern Europe in this century.

The weather in Europe is dominated and controlled by the low-pressure activity in the North-Atlantic. This activity is called the North Atlantic Oscillation (NAO) and can be expressed as an index, which is the difference in air pressure between the low-pressure areas south of Iceland and Greenland, and the high-pressure areas near the Azores. In particular during winter with strong low-pressures south of Iceland, which gives us a positive NAO-index, this leads to strong southwesterly winds, high temperatures and precipitation over Northern Europe. (For example: the relation between the precipitation in Bergen, Norway and the NAO-index important, with a correlation coefficient of 0.8). In Southern Europe and the Mediterranean area, we get drier weather and little precipitation under these conditions, see left figure. On the contrary, weaker low-pressure activity south of Iceland (negative NAO-index) leads to strong winds and heavy precipitation in Southern Europe and the Mediterranean, while Northern Europe gets northerly cold winds and dry weather, see figure on next page.

The North-Atlantic Oscillation is a natural phenomenon, which is particular dominant during winter, but is very difficult to predict from one year to another. For example, the 1990s had in general a positive NAOindex, which meant strong winds and heavy precipitation in Northern Europe. Suddenly, the NAO-index turned from positive (left figure) to negative (right figure) values during the winter of 1995/1996, from strong winds and rain to dry and cold weather in Northern Europe, without being predicted by the meteorological institutes in Europe and North-America. The North-Atlantic Oscillation has been present in several thousands of years, at least as long as the Norwegian Sea has been ice-free. One of the fundamental questions we may ask within climate research is how will increasing anthropogenic release of greenhouse-gases affect the natural weather systems, in our case, the North-Atlantic Oscillation?

Several international research institutes are currently working intensely with these problems. A group from the Nansen Centers in St. Petersburg, Russia and Bergen, Norway, Geophysical Institute and the Bjerknes Center at the University of Bergen, Norway, and the Max-Planck Institute for Meteorology, Hamburg, Germany, lead by Prof. Ola M. Johannessen have recently (February 17, 2005) published a scientific paper: The North-Atlantic Oscillation and greenhouse-gas forcing (www.nersc.no/MACESIZ/kuz05.pdf), in the well-known international scientific journal Geophysical Research Letters, American Geophysical Union, where this subject is treated. By participating in the international project, Coupled Model Intercomparison Project (CMIP2), we had access to the results from 12 global climate models from the leading climate institutes in the world. In order to have faith in the different models, we first made a comparison between models and observations, and the models were able to reproduce the observed distribution of air pressure, in a satisfactory way. The next step was to make two simulations of 80 years, the first with constant (at present level) atmospheric CO2 content and secondly with an atmospheric CO2 content increasing by 1% each year, approximately to the level we might expect at the end of this century, that is the double of today's atmospheric CO2 content. The first simulation gives us the natural variation of the NAO-system, while the second one gives us the natural variation and the effect of increasing release of greenhouse-gases. By comparing the two simulations and calculating the difference in the linear trends between these two simulations, we can indicate the potential influence increasing release of greenhouse-gases to the atmosphere has on the NAO weather-system.

Firstly, our results show that the observed NAO-trend we have experienced during the last decades, where we have a considerable increase in greenhouse-gases released to the atmosphere, is clearly higher than the simulations with constant CO2. This means that we already now observe an increase in the NAO-index under increasing greenhouse-gases. Furthermore, for future simulations, 8 of 12 models show a considerable increase (30-40%) in the NAO-index under increasing greenhouse-gases. Increasing NAO-index means increased low-pressure activity, which in turn leads to warmer weather, more precipitation and more storms. It should be mentioned that this result is based on simulations 80 years ahead in time, but since we analyse 12 climate models from the best climate research institutes worldwide, the results still give a good indication of what will happen with the low-pressure activity (the NAO-weather system) in this century. If "our prediction" is valid, it will have major positive and negative consequences. Some of this might be:

	Increasing precipitation in Northern Europe should lead to cheaper hydropower-based electricity
	A warmer climate with longer growth-seasons, and lower heating costs
	The ice in the Barents Sea will be pushed north- and eastwards due to increasing westerly winds and higher temperatures. This will expand fishing areas during winter and make it easier for oil- and gasexploration
	Increased frequency of storms will have major negative effects for sea-transport, fisheries, along with the entire coastal population
	Warmer weather and more precipitation may lead to increase danger of avalanches and flooding.

It is therefore important to focus research on how releasing greenhousegases in to the atmosphere will influence the natural climate variations, and to increase our understanding in order to better predict this in the future.

Kuzmina, S.I., L. Bengtsson, O.M. Johannessen, H. Drange, L.P. Bobylev and M.W. Miles (2005) The North Atlantic Oscillation and greenhouse-gas forcing, Geophysical Research Letters, 32(L04703), doi:10.1029/2004GL021064

For more information: Ola.Johannessen@nersc.no