Projection For The Major Components Of The Cryosphere Over The Northern Hemisphere In The 21St Century

Cryosphere is a major component of climate system. With the background of global warming, cryosphere has changed significantly. According to the IPCC Fifth Assessment Report, global temperature will persistently increase in future, which will lead to changes in cryopshere. In this paper, with the CMIP5 simulation and numerical models, the spatial and temporal changes in the major components of cryosphere with snow and frozen ground, and the net primary production (NPP) of terrestrial ecosystem in cold regions over the Northern Hemisphere are projected in the different periods (early period:2016-2035, middle period:2046-2065, late period:2080-2099) of 21st century, and the possible mechanism of their changes are also discussed.The multi-model ensemble from CMIP5 suggests that there is a significant decrease in snow water equivalent (SWE) for most regions over the Northern Hemisphere in the 21st century, particularly over the Tibetan Plateau (TP) and North America, but with an increase in Siberia; For three Representative Concentration Pathways (RCP2.6, RCP4.5, RCP8.5), the absolute change in annual mean SWE significantly occurs at the high latitude, with a significant relative change at the low latitude, which is due to the decrease of strong snowfall; Seasonal SWE projections show an overall decreasing trend for three RCPs, with a significant absolute change in winter, while relative change in SWE significantly occurs in summer; The decrease in SWE over the Northern Hemisphere is mainly linked to the decrease in snowfall before May, and the decrease in accumulated snowfall becomes the main cause of reduced SWE after May; In the spatial scale, the increased snowfall in Siberia is the main contributor to the increase in March mean SWE, while the increased melting associated with an earlier onset of spring lead to a decrease in SWE over the TP, North America and western Europe.With the modified Kudryavtsev model and the high resolution dataset of soil moisture(SM) from Global Land Data Assimilation System (GLDAS), the permafrost extent and active layer thickness (ALT) over the Northern Hemisphere in the 21st century are estimated. The results show that, the permafrost extent over the Northern Hemisphere in the 21st century represents a decreasing trend. Under RCP2.6 and RCP4.5, the permafrost extent shows insignificant change, while for RCP8.5, especially during the late period of 21st century, the permafrost in most regions will completely disappear, and only exists in Canada arctic, northern Russian, and the hinterland of Tibet Plateau. However, ALT shows an increasing trend, and the most significant increase in ALT occurs over the TP.In three typical permafrost regions (TP, Europe, North America (NA)), the contribution of SM, snow depth (SND) and mean annual air temperature (MAAT) to ALT are also discussed. The results show that, during 1986-2005, the significant contribution of SM to ALT only occurs over the TP, SND significantly correlates with ALT only in Europe, while MAAT significantly contributes to ALT in the three permafrost regions. Relative to the reference period of 1986-2005, ALT shows an increasing trend under the different periods and scenarios, while the soil thermal and hydraulic varies regionally. For the mid-high emissions (RCP4.5, RCP8.5), MAAT plays a dominant role in ALT over the TP, however, the effect of SND and SM on ALT is enhanced under the mid-low emission (RCP2.6, RCP4,5). In NA, MAAT has significant contribution to ALT only under RCP2.6 and RCP8.5. With the temperature increasing, snow begins to melt and cause the impact of snow to ALT gradually declines. In Europe, due to the Mediterranean climate, the soil thermal and hydraulic regime completely differ with that in NA and TP, the significant contribution of SND and MAAT to ALT only occurs during 1986-2005, and their impact on ALT gradually decreases in the 21st century. In general, MAAT is the main contributor to ALT during the 21st century, while the contribution of SM and SND to ALT gradually declines with the increasing temperature.With the increasing temperature, the permafrost thermal state has shown a significant response. Mean annual ground temperature (MAGT) over Northern Hemisphere in the 21st century shows an increasing trend, with a decreasing trend for the depth of zero annual amplitude (DZAA), and the most significant response of permafrost thermal regimes to climate change occurs in Siberia, TP, Canada arctic and Alaska, the changes of permafrost thermal state under RCP8.5 is greater than that under RCP2.6 and RCP4.5; The changes of permafrost in the 21st century mainly depend on the changes of cold permafrost; the changes of MAGT and DZAA are mainly related to the temperature increasing in winter, especially in January.With the climate warming, the terrestrial ecosystem also has significant changes. Relative to the reference period of 1986-2005, NPP in the mid-high latitude of Northern Hemisphere represents an increasing trend during the 21st century, and the increase of NPP under RCP8.5 is greater than that under RCP2.6 and RCP4.5; For the seasonal changes, NPP in the mid-high latitude of Northern Hemisphere also shows an increasing trend, the most significant increase of NPP appears in summer, especially in June; Furthermore, the response of NPP to climate change varies regionally, relative to the reference period of 1986-2005, for the middle and low scenarios (RCP2.6, RCP4.5), the range of significant impact of temperature on NPP in the mid-high latitude of Northern Hemisphere during the 21st century gradually narrows, while that of radiation and precipitation is gradually expanded. Under RCP8.5, the change of NPP in the mid-high latitude of Northern Hemisphere during the 21st century is mainly linked to temperature.