7 - Grasslands and the Global Carbon Cycle: Modeling the Effects of Climate Change

Summary

Abstract

The potential effects of climate change on grassland carbon budgets have received much less attention than those on forests. Changes in productivity and soil carbon need to be addressed, as well as the role of grassland burning in the global carbon cycle. Under a recent Scientific Committee on Problems of the Environment (SCOPE) collaborative project, we analyzed the sensitivity of global grassland ecosystems to modified climate and atmospheric CO₂ levels, concentrating on ecosystem dynamics rather than redistribution of grasslands. Worldwide, 31 grassland sites, temperate and tropical, were modeled under doubled-CO₂ climates projected by two different general circulation models. Results for climate change alone (without CO₂ effect) indicate that simulated soil C losses occur in most grassland regions, ranging from 0 to 14% of current soil C levels for the surface 20 cm. Direct CO₂ enhancement effects on decomposition and plant production tended to reduce the net impact of climate alterations alone. Detecting all these impacts will require a minimum of 16% change in net primary production and a 1% change in soil carbon. It is unclear whether grassland carbon stocks will increase overall under the new regimes, since changes in land use and biome area probably will result in a net source of carbon.

Introduction

Grassland ecosystems are far from uniform. They support myriad different species, both above and below ground, and are crucial in maintaining the ecological balance of a large part of the world, supporting diverse human and animal populations.