A study on changes in the vegetation and land surface dryness in present and future climate

Abstract

Historical observations show various responses to global warming over the land surface, one of important elements of Earths climate system as well as living place of humanity. Among those responses over the land, both changes in vegetation and land surface dryness are regarded as two major phenomena. Vegetation, occupies about 70% of whole land surface, is a key element of both physical and chemical processes over the land surface. Exact understanding of the vegetation change and its feedback influences on climate is essential for both investigating observed climate change and projection of future climate. Changes in land surface dryness are invisible, but important for the hydrological condition over the land, largely influences on agriculture and water management. Thus, researches on changes in both vegetation and land surface dryness contribute to mitigate risks of climate change because of both vegetation and land surface dryness has numerous socio-economic impacts on society. The present dissertation provides remarkable results of three studies about changes in vegetation and land surface dryness. First, the potential impact of vegetation feedback on land surface dryness in summer season is examined in a condition of doubling of atmospheric CO2 concentration over the contiguous United States (US) using a set of 100-year-long climate simulations integrated by global climate model (GCM) interactively coupled with a dynamic vegetation model. The Thornthwaite moisture index (Im), which quantifies land surface dryness on the basis of atmospheric water supply (i.e. precipitation, P) and atmospheric water demand (i.e., potential evapotranspiration, PET), is used to measure changes in the surface dryness. Warmer atmosphere and drier surface resulting from increased CO2 concentration increase land surface dryness over most of the contiguous US. This phenomenon is due to larger increments in PET than in P, regardless of the presence or absence of vegetation feedback. Compared to simulation without active dynamic vegetation feedback, the presence of vegetation feedback significantly alleviates the increase in land surface dryness. This vegetation-feedback effects is most notable in the subhumid regions such as southern, Midwestern and northwestern US, primarily by the increasing vegetation greenness. In these regions, the greening in response to warmer temperatures enhances moisture transfer from soil to atmosphere by evapotranspiration (ET). The increased et and subsequent moistening over land areas result in weaker surface warming (1–2 K) and PET (3–10 mm month–1), and greater P (4–10 mm month–1). Collectively, changes in temperature, PET, and P due to vegetation feedback result in moderate increases in Im, indicating decrease in land surface dryness. Next, the change in vegetation due to global warming is examined in detail focusing on its speed during twenty-first century using both a definition of plant habitat based on surface temperature and climate scenarios from multiple GCMs. The plant habitat changes are predicted by driving the bioclimate rule in a dynamic global vegetation model using the climate projections from 16 coupled GCMs. The timing of plant habitat change is estimated by the first occurrence of specified fractional changes (10%, 20%, and 30%). All future projections are categorized into three groups by the magnitude of the projected global-mean land surface temperature changes: low (<2.5K), medium (2.5-3.5K), and high (>3.5K) warming. During the course of the twenty-first century, dominant plant habitat changes are projected in ecologically transitional (i.e., from tropical to temperate and temperate to boreal) regions. The timing of plant habitat changes varies substantially according to regions. In the low-warming group, habitat changes of 10% in southern Africa occur in 2028, earlier than in the Americas by more than 70 yrs. Differences in the timing between regions increase with the increase in warming and fractional threshold. In the sub-tropics, fast plant habitat changes are projected for the Asia and Africa regions, where countries of relatively small gross domestic product (GDP) per capita are concentrated. Ecosystems in these regions will be more vulnerable to global warming, because countries of low economic power lack the capability to deal with the warming-induced habitat changes. Causes of changes in land surface dryness are not clear due to various attributions of climate variables on dryness changes. For exact understanding on complex spatial variability of land surface dryness changes, relative influences of five climate variables on dryness changes are quantified over continental East Asia, covering diverse hydro-climate regimes from humid to arid regions, by using observations from 189 stations for the period of 1961-2010. For the whole analysis period, the land surface dryness is decreased by both increasing P and decreasing PET, but the increasing trend is not monotonic. Since early 1980s, increasing trend of the land surface dryness is shown over monsoon climate area (> 100°E), but different climate variables drive the drying trend in each hydro-climate regimes. Dryness increases over the arid region are mostly explained by decrease in precipitation. In the humid area, increasing saturation vapor pressure following warming primarily contributes to dry surface despite continuous increase in precipitation. These results suggest increased evaporative potential, the secondary impact of atmospheric warming, plays a considerable role in changes in land surface dryness over the humid area even though sufficient atmospheric moisture exists at there. Conclusions of the present thesis suggest three meaningful implications. 1) Moistening by enhanced vegetation feedback may prevent aridification under climatic warming especially in areas vulnerable to climate change, with consequent implication for mitigation strategies. 2) The spatial distribution of plant habitat is projected to change quickly over countries of low economic power located on Asia and Africa. It is important to establish international collaboration via which developed countries provide assistance to mitigate the impacts of global warming. 3) The global warming sharply increases atmospheric water demands, inducing the risk of drying out over the land surface. Water management plans should consider the ongoing trend of drying accompanied by warming to mitigate the water scarcity in future.