Future Impacts of Climate Change on Forests
Even though the extent of future climate change remains largely uncertain, forests will be impacted to a certain degree under all future emissions scenarios.
In the ‘Future Environmental Impacts and Vulnerabilities’ chapter of the 2009 global assessment report of the International Union of Forest Research Organizations (IUFRO), scenarios are grouped into four clusters on the basis of recent emission patterns: unavoidable, which can be used to assess minimal adaptation needs; stable, in which greenhouse gas concentrations approach a new equilibrium by 2100; and growth and fast growth, which correspond to business-as-usual emissions, fast growth representing developments since 2000, which involve unprecedented high emission levels.
Under most scenarios, climate change is projected to change the distribution of forest types and tree species in all biomes. Globally, under growth and fast growth scenarios, all forest ecosystems will have difficulty adapting to the impacts of climate change. Forest ecosystem services are expected to be significantly altered, particularly in submesic, semi-arid and arid climates, where productivity could decline to the extent that forests are no longer viable. Several projections indicate significant risks that current carbon regulating services will be entirely lost, as land ecosystems turn into a net source of carbon beyond a global warming of 2.5°C (upper stable scenarios and beyond) or more relative to preindustrial levels. Moreover, since forests also release large quantities of carbon if deforested or impacted by other degrading stressors, they exacerbate climate change further.
Under scenarios in the stable and unavoidable scenario clusters, productivity levels in currently temperature-limited or humid climates will stay constant or even increase. Nevertheless, species compositions are projected to be altered significantly: e.g. from boreal to mixed-deciduous, from mixed deciduous to deciduous, from deciduous to savanna, or from boreal forest to grassland. Under most scenarios, boreal forests will be particularly affected by climate change and they are eventually expected to shift poleward. There are major uncertainties, however, regarding the time required for this shift. Under stable and growth scenarios, forest productivity is expected to generally increase at the northern end of the biome but, under scenarios beyond stable, to decrease in the currently more productive southern forests due to the impacts of insects and fire, leading to large carbon emissions that will exacerbate climate change. Under most scenarios, the temperate forests are likely to be less affected than other forest types by climate change. Large regional risks remain, however. Productivity is likely to increase in temperate forests closest to the poles and to decrease in temperate forests bordering the subtropics. Increasingly prevalent storms could cause major disturbances. Under growth scenarios, productivity in some subtropical woodlands could increase due to the fertiliser effect of higher atmospheric CO2 levels but, in other cases, rising temperatures, higher evaporation and lower rainfall could result in lower productivity. Droughts are projected to become more intense and frequent in subtropical and southern temperate forests, especially in the western United States, northern China, southern Europe, the Mediterranean and Australia. These droughts will also increase the prevalence of fire and predispose large areas of forest to pests and pathogens. In the subtropics the trend of increased fire is projected to wane in the latter part of the current century as lower rainfall reduces the availability of grass fuel. The subtropical domain contains many biodiversity hotspots that are at particular risk, even under scenarios stable. The productivity of tropical forests is projected to increase where water is sufficiently available; in drier tropical areas, however, forests are projected to decline. Tropical forests, particularly rain forests, harbour the highest biodiversity of all land ecosystems; even moderate climate change (such as that projected in unavoidable and stable scenarios) would put some of this biodiversity at a considerable risk. According to the IPCC, roughly 20–30% of vascular plants and higher animals on the globe are estimated to be at an increasingly high risk of extinction as temperatures increase by 2–3°C above pre-industrial levels. The estimates for tropical forests exceed these global averages. It is very likely that even more modest losses in biodiversity would cause consequential changes in ecosystem services.