Non-Amazonian South America has one of the highest rates of conversion of native ecosystems globally. Most of the studies investigating the climate impacts of these changes focus on the Amazon while the possible influences that these changes may have on climate of non-Amazonian regions have received less attention. The aim of this thesis was to evaluate the impacts of land use and land cover on the mean and extreme climate of non-Amazonian South America by conducting modelling experiments for pre-clearing (before year 1500) and present (year 2005) land covers. It develops new data sets of changes in land surface characteristics for this period and applies a high resolution regional climate model to simulate the potential impacts of changes in natural vegetation cover.
The thesis begins by providing a theoretical framework of surface-atmosphere interactions. It then reviews the process of land use and land cover change and subsequent climatic consequences in non-Amazonian South America and identifies those ecosystems most affected and least studied. The review highlights that non-Amazonian regions have lost more than 3.4 million km2 of natural vegetation since European colonization. Despite the magnitude of ecosystem loss, non-Amazonian South America accounts for significantly fewer studies addressing land surface-atmospheric processes compared to the Amazon region, highlighting the knowledge gap in relation to the main topic of this thesis. Based on these results, the following chapters address this knowledge gap by focusing on the climatic impacts of land use and land cover change in four main ecosystems: the Atlantic Forest (Brazil, Argentina and Paraguay), Cerrado (Brazil), Dry Chaco (Argentina, Bolivia and Paraguay) and the Chilean Matorral (Chile). I then apply the variable resolution CSIRO Conformal Cubic Atmospheric Model (CCAM) global climate model at a 25 km spatial resolution over the South American continent to quantify the seasonal climate impacts of each of historic land cover change.
The results of computed modelling experiments show significant changes in surface fluxes, temperature, precipitation and moisture in all ecosystems. For instance, simulated temperature changes were stronger in the Cerrado and the Chilean Matorral with an increase of between 0.7 and 1.4 °C. Changes in the hydrological cycle revealed high regional variability. Results also show that the loss of natural vegetation has significantly affected temperature extremes as a decrease in the number of warm days and an increase in the number of warm nights. Importantly, there is a strong dependence on both seasonality and the vegetation contrast inflicted by land use/cover change, with large roughness changes resulting in increasing wind speed and advection, while smaller roughness changes result in feedbacks more reliant on the distinction between sensible and latent heat fluxes. This explains the dry season response in both temperature extremes and the increase in aridity according to land use/cover change, whereby regions with increased wind speed reduce warm day temperature extremes, despite increasing mean temperature trend, and have a greater impact on atmospheric water demand than those regions that mainly increase sensible heat fluxes. These results can explain the observed trends in temperature extremes in non-Amazonian South America and highlights the need to embed land use/cover change as a forcing within future climate change scenarios.
The main conclusions of the thesis are: a) non-Amazonian South America is one of the most impacted and least studied regions worldwide in terms of climatic impacts of land use and land cover change, b) modelled impacts of this process are expressed as significant changes in surface temperature and the hydrological cycle through changes in soil and atmospheric moisture, and have increased the aridity in all the examined ecosystems. The thesis highlights the importance of considering the influence of land use and land cover on the mean and extreme climate through changes in biophysical properties that significantly impact the surface-atmospheric coupling and therefore the hydrological cycle. This is critical consideration for national natural ecosystem management strategies as conserving and restoring natural vegetation cover may help mitigate the negative consequences of climate change and therefore have a direct influence on the welfare of the region’s 200 million inhabitants.