Impacts of climate change on soils and land systems in New Zealand

About 65% of New Zealand's economy is based on its natural and managed land systems and soils. Present understanding of their relative sensitivity to climate change is generally poor and largely qualitative. This paper examines the possible impacts of an enhanced greenhouse effect, with particular emphasis being given to identifying the most sensitive land systems and soils. A scenario of climate change, based on the IS92a emissions scenario of the IPCC, is used. The sensitivity of land systems is examined in two ways, Firstly, a first-order indication of how agroecosystems might respond to climate change is illustrated by assessing possible spatial changes in grain maize suitability. Secondly, susceptibility of land to erosion, with possible concomitant losses in productivity is assessed against changes in precipitation. The analysis suggested only small changes in erosion severity appear likely except in critical areas where a substantial increase could result from increases in storm frequency and intensity. In evaluating the sensitivity of soils, the emphasis is on soil organic matter. Soil organic matter, which stabilises topsoils and stores most of the nutrients, is critically important for the sustainability of land systems. Spatially referenced data for land use, soil carbon and climate are used in a model to indicate the possible responses of organic matter to climate change for the major ecosystems of New Zealand. Reductions in soil carbon of about 5-6% per degree temperature increase were generally expected in the absence of effects from carbon dioxide fertilization and increased nutrient availability. Soils likely to reach critical minimum-threshold concentrations of organic matter over the next 60 years are identified.