Climate effects of land use changes and anthropogenic impact on surface radiation

The fourth assessment report on climate change (AR4) was released in 2007 and the Intergovernmental Panel of Climate Change (IPCC) derive an increase of 0.74 ± 0.18°C in the 100 year global mean surface temperature linear trend between 1906 – 2005. IPCC state further that “there is very high confidence that the global average net effect of human activities since 1750 has been one of warming” (IPCC, 2007). The observed global warming has occurred during the same period as a considerable increase of greenhouse gases in the atmosphere. The most important anthropogenic greenhouse gas is carbon dioxide (CO2) and the global concentration of CO2 has increased by 35% from 280 parts per million (ppm) since pre industrial times to 379 ppm in 2005. The sources of increasing CO2 in the atmosphere are related to human activity through fossil fuel combustion and deforestation. Other important greenhouse gases that have increased since pre industrial times are N2O and CH4 and their sources are primarily from agriculture. At the same time as the observed global warming, there has been an increasing amount of anthropogenic emissions of particles into the atmosphere. These particles are known as aerosols and have mainly caused global cooling by scattering and absorbing solar radiation. It is proposed that the aerosols have partly masked the warming caused by the greenhouse gases (Charlson et al., 1992,Ramanathan et al., 2001,Kaufman et al., 2002,Chung et al., 2005)

Radiative forcing by changes in surface albedo caused by changes in vegetation

The human influence on vegetation causes changes in the surface reflective properties. By using MODIS land cover and MODIS surface albedo products, an estimation of radiative forcing due to surface albedo changes caused by vegetation changes is performed. A potential natural vegetation data set is used to compute radiative forcing estimates from pre agricultural times to present. A combination between MODIS blacksky and whitesky albedo and diffuse and direct radiation at ground level makes it possible to improve the accuracy of the present surface albedo. A new self-composed surface albedo data set is calculated for the purpose of not overestimating the radiative forcing in snow covered cropland regions. For that reason, a constraint on the pre agricultural data set is carried out by not allowing any surface albedo values to be lower than 0.081. The best estimate shows a radiative forcing due to the surface albedo change of -0.03 W/m2, which is weaker than what has been claimed earlier by previous studies. This is mainly because of a more realistic value of cropland, the albedo constraint, and also the intrinsic power of the method consisting of combining two MODIS products

Climate warming feedback from mountain birch forest expansion: Reduced albedo dominates carbon uptake

Expanding high elevation and high latitude forest has contrasting climate feedbacks through carbon sequestration (cooling) and reduced surface reflectance (warming), which are yet poorly quantified. Here, we present an empirically-based projection of mountain birch forest expansion in south-central Norway under climate change and absence of land use. Climate effects of carbon sequestration and albedo change are compared using four emission metrics. Forest expansion was modeled for a projected 2.6 °C increase of summer temperature in 2100, with associated reduced snow cover. We find that the current (year 2000) forest line of the region is circa 100 m lower than its climatic potential due to land use history. In the future scenarios, forest cover increased from 12 to 27% between 2000 and 2100, resulting in a 59% increase in biomass carbon storage and an albedo change from 0.46 to 0.30. Forest expansion in 2100 was behind its climatic potential, forest migration rates being the primary limiting factor. In 2100, the warming caused by lower albedo from expanding forest was 10 to 17 times stronger than the cooling effect from carbon sequestration for all emission metrics considered. Reduced snow cover further exacerbated the net warming feedback. The warming effect is considerably stronger than previously reported for boreal forest cover, because of the typically low biomass density in mountain forests and the large changes in albedo of snow-covered tundra areas. The positive climate feedback of high latitude and high elevation expanding mountain forests with seasonal snow cover exceeds those of afforestation at lower elevation, and calls for further attention of both modelers and empiricists. The inclusion and upscaling of these climate feedbacks from mountain forests into global models is warranted to assess the potential global impacts