M. M. Arzhanov

Interaction of the Methane Cycle and Processes in Wetland Ecosystems in a Climate Model of Intermediate Complexity

The climate model of the Institute of Atmospheric Physics of the Russian Academy of Sciences (IAP RAS CM) has been supplemented with a module of soil thermal physics and the methane cycle, which takes into account the response of methane emissions from wetland ecosystems to climate changes. Methane emissions are allowed only from unfrozen top layers of the soil, with an additional constraint in the depth of the simulated layer. All wetland ecosystems are assumed to be water-saturated. The molar amount of the methane oxidized in the atmosphere is added to the simulated atmospheric concentration of CO2. A control preindustrial experiment and a series of numerical experiments for the 17th–21st centuries were conducted with the model forced by greenhouse gases and tropospheric sulfate aerosols. It is shown that the IAP RAS CM generally reproduces preindustrial and current characteristics of both seasonal thawing/freezing of the soil and the methane cycle. During global warming in the 21st century, the permafrost area is reduced by four million square kilometers. By the end of the 21st century, methane emissions from wetland ecosystems amount to 130–140 Mt CH4/year for the preindustrial and current period increase to 170–200 MtCH4/year. In the aggressive anthropogenic forcing scenario A2, the atmospheric methane concentration grows steadily to ≈3900 ppb. In more moderate scenarios A1B and B1, the methane concentration increases until the mid-21st century, reaching ≈2100–2400 ppb, and then decreases. Methane oxidation in air results in a slight additional growth of the atmospheric concentration of carbon dioxide. Allowance for the interaction between processes in wetland ecosystems and the methane cycle in the IAP RAS CM leads to an additional atmospheric methane increase of 10–20% depending on the anthropogenic forcing scenario and the time. The causes of this additional increase are the temperature dependence of integral methane production and the longer duration of a warm period in the soil. However, the resulting enhancement of the instantaneous greenhouse radiative forcing of atmospheric methane and an increase in the mean surface air temperature are small (globally < 0.1 W/m2 and 0.05 K, respectively).

Simulation of Characteristics of Thermal and Hydrologic Soil Regimes in Equilibrium Numerical Experiments with a Climate Model of Intermediate Complexity

The IAP RAS CM (Institute of Atmospheric Physics, Russian Academy of Sciences, climate model) has been extended to include a comprehensive scheme of thermal and hydrologic soil processes. In equilibrium numerical experiments with specified preindustrial and current concentrations of atmospheric carbon dioxide, the coupled model successfully reproduces thermal characteristics of soil, including the temperature of its surface, and seasonal thawing and freezing characteristics. On the whole, the model also reproduces soil hydrology, including the winter snow water equivalent and river runoff from large watersheds. Evapotranspiration from the soil surface and soil moisture are simulated somewhat worse. The equilibrium response of the model to a doubling of atmospheric carbon dioxide shows a considerable warming of the soil surface, a reduction in the extent of permanently frozen soils, and the general growth of evaporation from continents. River runoff increases at high latitudes and decreases in the subtropics. The results are in qualitative agreement with observational data for the 20th century and with climate model simulations for the 21st century.

Changes in climatic characteristics of Northern Hemisphere extratropical land in the 21st century: Assessments with the IAP RAS climate model

Assessments of future changes in the climate of Northern Hemisphere extratropical land regions have been made with the IAP RAS climate model (CM) of intermediate complexity (which includes a detailed scheme of thermo- and hydrophysical soil processes) under prescribed greenhouse and sulfate anthropogenic forcing from observational data for the 19th and 20th centuries and from the SRES B1, A1B, and A2 scenarios for the 21st century. The annual mean warming of the extratropical land surface has been found to reach 2–5 K (3–10 K) by the middle (end) of the 21st century relative to 1961–1990, depending on the anthropogenic forcing scenario, with larger values in North America than in Europe. Winter warming is greater than summer warming. This is expressed in a decrease of 1–4 K (or more) in the amplitude of the annual harmonic of soil-surface temperature in the middle and high latitudes of Eurasia and North America. The total area extent of perennially frozen ground Sp in the IAP RAS CM changes only slightly until the late 20th century, reaching about 21 million km2, and then decreases to 11–12 million km2 in 2036–2065 and 4–8 million km2 in 2071–2100. In the late 21st century, near-surface permafrost is expected to remain only in Tibet and in central and eastern Siberia. In these regions, depths of seasonal thaw exceed 1 m (2 m) under the SRES B1 (A1B or A2) scenario. The total land area with seasonal thaw or cooling is expected to decrease from the current value of 54–55 million km2 to 38–42 in the late 21st century. The area of Northern Hemisphere snow cover in February is also reduced from the current value of 45–49 million km2 to 31–37 million km2. For the basins of major rivers in the extratropical latitudes of the Northern Hemisphere, runoff is expected to increase in central and eastern Siberia. In European Russia and in southern Europe, runoff is projected to decrease. In western Siberia (the Ob watershed), runoff would increase under the SRES A1B and A2 scenarios until the 2050s–2070s, then it would decrease to values close to present-day ones; under the anthropogenic forcing scenario SRES B1, the increase in runoff will continue up to the late 21st century. Total runoff from Eurasian rivers into the Arctic Ocean in the IAP RAS CM in the 21st century will increase by 8–9% depending on the scenario. Runoff from the North American rivers into the Arctic Ocean has not changed much throughout numerical experiments with the IAP RAS CM.

Impact of Global Warming Rate on Permafrost Degradation

The IAP RAS climate model of intermediate complexity is used to analyze the sensitivity of the area of continuous potential permafrost Scont to the rate of global temperature variation Tgl in experiments with greenhouse-gas increases in the atmosphere. The influence of the internal variability of the model on the results is reduced by conducting ensemble runs with different initial conditions and analysis of the ensemble means. Idealized experiments with a linear or exponential dependence of the concentration of carbon dioxide in the atmosphere have revealed an increase in the magnitude of the temperature-sensitivity parameter of the area of continuous potential permafrost, kcont (= Scont, 0t-1dScont/dTgl, where Scont, 0 is the present value of Scont). With a decrease in the linear trend coefficient of Tgl from about 3 to about 2 K/100 yr, this parameter varies from approximately −0.2 to −0.4 K−1. With an even slower change in global temperature, kcont virtually does not vary and remains close to the value obtained from paleoreconstructions of the past warm epochs. Such a dependence of kcont on the rate of global warming is related mainly to the fact that the more rapid increase in Tgl leads to a slower response over high-latitude land. The contribution from changes in the annual temperature cycle, though comparable in the order of magnitude, is about one-third as large as the contribution from the variation of the latitudinal structure of the response of annual mean temperature. The total reduction in the annual cycle of temperature during warming partly compensates for the effect of the annual mean temperature rise, thus decreasing the magnitude of kcont. In numerical experiments with greenhouse gas changes in accordance with SRES scenarios A2 and B2 and scenario IS92a, there is also a monotonic increase in the magnitude of the normalized parameter of temperature sensitivity of the area of continuous permafrost with a decrease in the growth rate of global temperature. For scenarios A2-CO2, IS92a-GHG, IS92a-CO2, B2-GHG, and B2-CO2, its value is almost indistinguishable from the steady-state asymptotic value of −0.4 K−1. For A2-GHG, the magnitude of kcont turns out to be far less (kcont ≈ −0.3 K−1).

Estimating Climate Changes in the Northern Hemisphere in the 21st Century under Alternative Scenarios of Anthropogenic Forcing

Possible changes in the climate characteristics of the Northern Hemisphere in the 21st century are estimated using a climate model (developed at the Obukhov Institute of Atmospheric Physics (OIAP), Russian Academy of Sciences) under different scenarios of variations in the atmospheric contents of greenhouse gases and aerosols, including those formed at the OIAP on the basis of SRES emission scenarios (group I) and scenarios (group II) developed at the Moscow Power Engineering Institute (MPEI). Over the 21st century, the global annual mean warming at the surface amounts to 1.2–2.6°C under scenarios I and 0.9–1.2°C under scenarios II. For all scenarios II, starting from the 2060s, a decrease is observed in the rate of increase in the global mean annual near-surface air temperature. The spatial structures of variations in the mean annual near-surface air temperature in the 21st century, which have been obtained for both groups of scenarios (with smaller absolute values for scenarios II), are similar. Under scenarios I, within the extratropical latitudes, the mean annual surface air temperature increases by 3–7°C in North America and by 3–5°C in Eurasia in the 21st century. Under scenarios II, the near-surface air temperature increases by 2–4°C in North America and by 2–3°C in Eurasia. An increase in the total amount of precipitation by the end of the 21st century is noted for both groups of scenarios; the most significant increase in the precipitation rate is noted for the land of the Northern Hemisphere. By the late 21st century, the total area of the near-surface permafrost soils of the land of the Northern Hemisphere decreases to 3.9–9.5 106 km2 for scenarios I and 9.7–11.0 × 106 km2 for scenarios II. The decrease in the area of near-surface permafrost soils by 2091–2100 (as compared to 2001–2010) amounts to approximately 65% for scenarios I and 40% for scenarios II. By the end of the 21st century, in regions of eastern Siberia, in which near-surface permafrost soils are preserved, the characteristic depths of seasonal thawing amount to 0.5–2.5 m for scenarios I and 1–2 m for scenarios II. In western Siberia, the depth of seasonal thawing amounts to 1–2 m under both scenarios I and II.

Assessment of the response of subaqueous methane hydrate deposits to possible climate change in the twenty-first century

Large reserves of methane are trapped in oceanic hydrate deposits. Increase in temperature of the oceans makes a contribution to the dissociation of oceanic hydrate accumulations and release of poten� tially large amounts of methane into the atmosphere. Such releases into the atmosphere lead to an increase in the greenhouse effect and, accordingly, serious climate changes and to accelerate the gas hydrate dissociation. In this work we evaluated the sta� bility of the existing reserves of subaqueous (underwa� ter) gas hydrates and possible methane release with the dissociation of methane hydrates in the twentyfirst century (1). Methane hydrates are compounds in which meth� ane molecules are in cells formed by water molecules. They are widely distributed in the permafrost zones and the oceanic bottom sediments along the continental slopes, where they are stable at the current PTvalues. Methane hydrates are a potentially large source of energy in comparison with other known sources of hydrocarbons. The total carbon in hydrates is esti� mated as 10 4 Gton C (2), which is a significant amount in comparison with the carbon content (3.8 ×10 4 Gton C) dissolved in the oceanic water and in soil and plants (2 × 10 3 Gton C) and the atmosphere (7.3 ×10 2 Gton C) (3). The total fossil fuel reserves, including coal, are about 5 ×10 3 Gton C (4); i.e., they are consistent with gas hydrate reserves. Methane is the third (after water vapor and carbon dioxide) greenhouse gas, which has a significant effect on the radiation balance of Earths climate system and can be released into the atmosphere as a result of min� ing and use of hydrates as an energy source. Sudden releases of methane into the atmosphere may occur due to massive underwater shifts of the Earths crust and an increase in temperature in the oceanic bottom sediments. According to the model estimates, in case of an increase in the oceanic water temperature by a few degrees, methane hydrate reserves should be much smaller (5). Releases of methane in decomposition of methane hydrates could have been a cause of abrupt climate change in the past (6, 7). The Paleocene- Eocene temperature maximum is a wellknown exam� ple of a period of abrupt climate change that was likely associated with massive release of methane from hydrates 55 Ma ago. In some areas (including the Car� ibbean Sea, North Atlantic, the Weddell Sea, and tropical Pacific Ocean), a shift of 2.5- δ 13 С in biogenic carbonate and organic matter was noted. This may be associated with the release of 1500-2000 Gton of methane over several thousand years (6). Such a large release of methane could influence cli�

Temperature trends in the permafrost of the Northern Hemisphere: Comparison of model calculations with observations

We present the results of analysis of numerical calculations of the thermal state of permafrost grounds at different depths using a model of heat and moisture transport in the ground developed at the Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS). For high-latitude regions of Russia, the model-estimated temperature trends in grounds (around 0.3°C/10 years at a depth of 3 m) are quite consistent with empirical estimates for the past few decades.

Impact of climate changes over the extratropical land on permafrost dynamics under RCP scenarios in the 21st century as simulated by the IAP RAS climate model

Estimates of possible climate changes and cryolithozone dynamics in the 21st century over the Northern Hemisphere land are obtained using the IAP RAS global climate model under the RCP scenarios. Annual mean warming over the northern extratropical land during the 21st century amounts to 1.2–5.3°C depending on the scenario. The area of the snow cover in February amounting currently to 46 million km2 decreases to 33–42 million km2 in the late 21st century. According to model estimates, the near-surface permafrost in the late 21st century persists in northern regions of West Siberia, in Transbaikalia, and Tibet even under the most aggressive RCP 8.5 scenario; under more moderate scenarios (RCP 6.0, RCP 4.5, and RCP 2.6), it remains in East Siberia and in some high-latitude regions of North America. The total near-surface permafrost area in the Northern Hemisphere in the current century decreases by 5.3–12.8 million km2 depending on the scenario. The soil subsidence due to permafrost thawing in Central Siberia, Cisbaikalia, and North America can reach 0.5–0.8 m by the late 21st century.

Changes in the boundaries of the permafrost layer and the methane hydrate stability zone on the Eurasian Arctic shelf, 1950–2100

By using the model for subsea sediments (SSs) (Institute of Atmospheric Physics, Russian Academy of Sciences, IAP RAS) and the general circulation model in the Arctic Ocean–North Atlantic (GCM AO-NA) (Institute of Computational Mathematics and Mathematical Geophysics, Siberian Branch, Russian Academy of Sciences, ICMMG SB RAS), the response of the parameters of the permafrost layer and the methane hydrate stability zone (MHSZ) to external impacts in dependence on the parameters of the problem is considered: the degree of the geothermal heat flux intensity G at the lower (bottom) boundary of the computation domain of the permafrost layer of subsea sediments and the depth Z of this boundary.

Model assessments of organic carbon amounts released from long-term permafrost under scenarios of global warming in the 21st century

346 The general increase in the surface temperature recorded in recent decades, which is especially high at high and mid latitudes, and variations in the amount of precipitation influence the thermal and hydrologi cal regime of the planet. At the end of the 20th century, the rate of annual mean temperature increase in long term permafrost rocks was as high as 0.03°C per year based on observations in some land regions of the Northern Hemisphere [1]. It may increase in the 21st century up to 0.05°C per year based on the results of simulations using global climatic models [2]. As a result, we can expect an increase in the depth of the active layer with inclusion of organic matter into the biogeochemical cycle in the thawed permafrost, which can lead to the emission of greenhouse gases into the atmosphere. Degradation of surface long term per mafrost rocks facilitates release of greenhouse gases conserved in them and an increase in the positive feed back between the cryolitic zone and the atmosphere if the expected climate variations occur [3, 4].