by Igor MOKHOV, Corresponding Member of the Russian Academy of Sciences, RAS Institute of Physics of the Atmosphere named after A. Obukhov (IFA)

Changes of the climate-current and coming-are one of the major problems of our time which requires a comprehensive approach. That includes taking into account the impact of human activities. Studies of key problems involved are conducted by our Institute scientists in close cooperation with specialists from other research organizations of this country and foreign colleagues.

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Air temperature near the surface of the Earth during the past century was rising much faster than during the past 2000 years (according to paleoreconstructions). Naturally enough, one cannot disregard an important role in this process of natural climate variations caused by changes in solar radiation and volcanic activities. For example, the warming in the first half of the 20th century can be explained by exactly such reasons, but according to calculations in these past few decades the main role belonged to anthropogenic factors, including the central one-mounting levels in the atmosphere of hothouse gases, above all of carbon dioxide. As compared with what we call the "pre-industrial time", its level increased by one third. Man's intrusion into the "weather laboratory" of nature is demonstrated by temperature difference in different strata of the atmosphere. With growing intensity of solar radiation the temperature is increased not only of the lower layers, but also of the stratosphere and mesosphere. And with the growing hothouse effect the latter are cooling down and it becomes warmer only in the troposphere (up to 8 - 16 km from the Earth surface). This is exactly what happens now and the gas "envelope" of our planet is cooling down mostly near the menopause (at altitudes of some 80 - 90 km).

An important characteristic of climate changes are their rates, which are especially significant in high latitudes (Siberia, North America, Antarctica). According to the results of observations the rate of growth of the mean global near-surface temperature is growing. In 1970 - 2000, for example, it amounted to some 0.2 degrees/10 years, but for some regions of the planet its maximum values were several times greater.

Record values were registered on the border of last and present centuries. For example, in the Irkutsk Region and on the Alaska (Barrow) it reached 0.8 degrees/10 years and

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more. In Antarctica in the second half of the 20th century the rate of warming was 0.3 - 0.5 degrees/10 years and it became even greater over the past few years. The maximum most often occur during winter months, but in places like Alaska and Chukotka they also occur in spring.

We conducted our studies in this field in conjunction with our colleagues from the Headly Center of climate studies and prognostications (Great Britain) and we compared data of observations and the model of general circulation of the atmosphere and the ocean developed by the British. And we studied three scenarios: taking into account only natural impacts (fluctuation of solar radiation intensity and levels of stratospheric aerosol due to volcanic eruptions; only anthropogenic impacts (raising levels in the atmosphere of hothouse gases and sulfate aerosol) and both. The rates of warming calculated by the first scenario for regions of Siberia, North America and Antarctica are low and insignificant, and are close for the second and third ones.

What they call general correspondence of calculations and values registered by meteorologists have been obtained for Alaska and Antarctica. And the picture is much different for Siberia: in recent years of the past century much faster growth of temperature was registered there than data of calculations. We think that such differences are caused by an "overestimated" model of the effect of aerosol cooling. These data characterize the situation on territories of about 200 - 300 km, and observations reports referred to a concrete spot-in the area of the related station. On the whole KMOTS can adequately reflect the aforesaid changes.

What are the likely consequences of climate changes, for our country in particular? First of all they can impact the permafrost layers on the important factors of which is the sensitivity of the cryolite zone rising surface temperature. According to the global climate model developed at our Institute (with scenarios with different rates of growth of the levels of hothouse gases in the atmosphere) we analyzed this parameter and it turned out to be smaller for cases with bigger anthropogenic discharges and rapid changes of the climate.

The IFA RAS model makes it possible to describe eight layers of the atmosphere (up to 80 km), three-of oceanic block and two of dry land, it takes into account biospheric effects, changes of marine ice and reproduces well enough on the whole the present-day climate. The mean warming figure of 0.7 degrees for the past 100 years obtained from observations is within the range of these assessments. By the middle of the 19th century it is expected to increase by 1 - 1.5 degrees as compared with the basic regime of 1961 - 1990, and by the end of this century-by 1.5 - 3 degrees.

At the Climate Theory Laboratory of our Institute they studied yet another aspect of the problem of permafrost - the depth of seasonal thawing. As proved by calculations according to the model of thermophysical processes in the ground, during warming it may increase in Northern Eurasia, in Russia for example, by 20 - 40 percent. In some regions "taliks"* can be formed, and the moment of separation of permafrost from the active layer of soil will depend on the nature of the latter-with all other conditions being equal, it will occur earlier for sands and later for clays.

Working together with our German colleagues from the Hamburg Meteorological Institute of M. Plank (MIMP) and the Institute of Marine Sciences of the Kiel University we have been assessing the possible changes of various characteristics of natural precipitation for Northern Eurasia in the 20th-21st centuries in different scenarios. For calculations we used climatic models of MIMP and IFA RAS taking into account anthropogenic emissions of hothouse gases.

We analyzed in detail the hydrogeological regimes in the basins of the biggest rivers of Russia and in the Caucasus. In the present century for the regions of the Ob, Yenisei, Lena and Volga in winter time we can expect increases of the total amounts, intensity, extreme regimes and possibility of precipitation (river discharges will increase accordingly). In summer-time the first three factors are generally increased, but the last one decreases. A more variegated picture is observed in the Caucasian region with its closely neighboring arid zones and those with normal or increased humidity. According to data provided by 10 local weather stations, the tendencies for changes in the natural precipitation differ considerably (sometimes polarly) in the northern and southern, maritime and more continental regions. Therefore the results of the calculations often depend considerably from the choice of the borders of the investigates territories and scenarios of climate changes on our planet.

In general with the arrival of a global warming one can expect over many regions, including Northern Eurasia in winter time, considerable growth of extreme values and mean intensities of precipitation. In more southern regions their probability in any time of the year will decrease.

The results obtained according to the German and Russian models are generally in agreement (especially for hibernal regional regimes). For better description of meridional regimes we are planning further modification of our model.

And it should be noted that model assessments of changes of winter precipitation are more stable (meridional) than of summer ones. This can be explained by intensive changes of the temperature regime (according to observations and model assessments) and intense atmospheric circulation in cold months. During that time of the year moisture transfer is stronger from west to east, including that over Northern Eurasia, and more apparent regional climatic regularities which are authentically reproduced by even relatively simple models.

Regional peculiarities of the temperature and hydrological regimes are connected with what are called centers of atmospheric impact, including the Siberian and Azorean maximums (anticyclones), Icelandic and Aleutian mini-

* Talik - defrosted area of soil in areas of criolytoxon with positive temperature at least for part of а year. - Ed.

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Changes of winter precipitation (% in 100 years) in the 21st century (compared with period 1961 - 1990) with anthropogenic scenario by KMOTS calculations and climate model of IFA RAS.

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mums (cyclones). Calculations of their average parameters with the help of our model generally match the results of observations with the exception of some overestimations of pressure in the center of subtropical maximums. In the 21st century, according to scenarios of anthropogenic warming, one can expect for example, a weakening of the Siberian maximum and strengthening of the Aleutian minimum.

With the help of KMOTS and taking into account the carbonic cycle, we, together with specialists of the Paris Laboratory of Dynamic Meteorology, assessed possible changes of droughts regimes and the interconnected biological productivity of ground ecosystem in Northern Eurasia in the present century (taking into account anthropogenic activities). For testing the results of calculations we used weather forecasts, including data on mean monthly anomalies of near-surface temperatures and precipitations in periods of plants vegetation (May-July) from the end of the 19th century. The analysis revealed that the real and model data are generally in agreement. The latter revealed in the mean latitudes of European Russia an increase of about 10 percent of the spread of droughts with a rise of one degree of surface temperature. In future one can probably expect an intensification of this process during the autumn-summer period.

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Primary biological productivity due to climate changes linked with rises of mean annual global near-surface temperature by one degree (model of IFA RAS).

One of the key characteristics of biological productivity and agricultural potential is what we call net primary productivity which determines the intensity of carbon dioxide absorption from the atmosphere (exchange with continental biota). According to model calculations, in the 21st century its mean values and variability in the analyzed regions of Northern Eurasia will generally increase because of intensification of biosynthesis with growing levels of CO₂ in the air even in unfavorable weather conditions.

We have been convinced that the best contemporary models can adequately describe not only the main global and regional features of the global climate system, but also their changes. Nevertheless the uncertainty of the numerical assessments of the latter is not reduced. The reason for that above all are fundamental restrictions associated with the impossibility in principle of their accurate prognostication, an uncertainty of the likely scenarios of emissions of hothouse gases and aerosol. Apart from that most of the models do not use the carbonic cycle and make only a rough assessment of it, which reduces their temperature sensitivity. Many things still remain unclear in the problems of carbon exchange between the atmosphere and biosphere, dry land and the ocean. For example, it has been estimated that over the last several years Russian forests have been consuming more of this gas that before.

Another problem consists in an adequate consideration of the effects of the natural, including volcanic and anthropogenic aerosols because modeling of photochemical processes in the atmosphere, including troposphere, is important today not only for megapolises, but for the planet as a whole. At the same time our models should reproduce in reality extreme situations (extreme natural precipitation, floods, droughts, fires, etc.) on regional levels.

There are still many problems in modeling tropical hurricanes, detailed analysis of oceanic circulation under the impact of global warming and its consequences on certain territories. All this is closely linked with the need for a realistic description of major climate changes in the Arctic basin-changes of temperature, and sea ice dynamics. According to satellite measurements carried out since 1970s, the ice-shield area was minimal in 2005. One of what we call critical regimes is the replacement of the growth of the Antarctic ice-shield by the thawing of ice when a certain level of warming is exceeded. We also need some more detailed models of permafrost taking into account the impacts of the snow and moss covers.

A key role in the changes under consideration belongs to the cloud cover. Its decrease or increase substantially influences the sensitivity of the climate system to different influences, including anthropogen ones.

Finally, in order to deeper our understanding of complicated, diverse climate processes and their changes, we have to rely, together with global, also on regional models, including Russian ones-of the Main Geophysical Observatory and of the RAS Institute of Calculation Mathematics and IFA RAS.

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