Executive Summary

The final draft of the full report: Climate Change: the Threats to World Forests will be available shortly. Below is the executive summary taken from the report.

Over the 21st century, the temperature at the earth's surface is likely to increase significantly, and all ecosystems, including forests, will experience the most rapid period of climate change since the end of the last ice age. The distribution and composition of forests will be affected by this change, and effective conservation strategies will need to accommodate the prospect of rapidly migrating climate zones and shifting ecosystems.

Using standard current climate prediction scenarios, the key changes expected by the end of the 21st century are: the mean global temperature will increase by 1.5 - 4.5 °C; the atmospheric concentration of carbon dioxide will approximately double; precipitation will increase globally by 3-5%; and sea level will increase by about 45 cm.

Regional climate predictions are needed to determine climate change-related threats to forests. There is a fair degree of confidence in most regional temperature predictions. The largest increases in temperature will be in the northern high latitudes, with lower increases nearer the tropics and in regions with a strong oceanic influence. Although precipitation will increase globally, regional predictions are less certain, vary in the magnitude and sometimes the direction of the change. Overall, the key climatic changes controlling forest growth responses will be temperature increases at high latitudes and changes in rainfall at low latitudes. Any regions with increased temperature and unchanged or reduced rainfall will experience significant reductions in soil moisture.

The response of forests to these changes will depend on the differential capability of species to accommodate environmental changes by continuing to grow and reproduce, or by migrating to follow shifting climate zones. This capability will be determined by the variation within and among species in their physiological responses to changes in temperature, CO2 concentration, soil moisture, and, in some areas, increased nitrogen deposition. It will also depend critically on the ecological relationships between species that affect pollination, dispersal, and damage through herbivore or pest and pathogen attacks. The nature of the landscape and the intensity of human activities affecting forests will constrain these responses by affecting forest condition and patterns of species migration and establishment. Migration processes are likely to lead to new species assemblages and may involve significant species losses.

Boreal forests will experience the largest temperature increases of all forests. Warming will be greater in winter (4.0 °C above 1970s levels by the mid-21st century) and slightly lower in summer (2.5-3.0 °C above 1970s levels). Reduced moisture in the soil during summer will increase drought-stress and the extent of wild fires. Climate zones will migrate northwards at rates of up to 5 km per year. Most tree species are not able to migrate this fast. Boreal forests will make gains in area to the north, but will experience dieback and replacement at their southerly extremes. Vegetation models suggest that the overall response may be either a reduction (by up to 36%) or an expansion (by up to 16%) of boreal forest area, though a reduction is more likely. Few species are likely to become extinct, but local species loss rates may be high during migration phases.

Temperate forests will be affected most at higher latitudes by climate warming (2.6 °C above 1970s levels by the mid-21st century) and at lower latitudes by changes in rainfall. Drought-stress at certain low-latitude margins will lead to significant die-back, whilst increased temperature will enhance growth and establishment of temperate forest species at higher latitudes. An overall expansion of potential temperate forest area is likely (by between 7% and 58%). The relatively high level of fragmentation of many temperate forests is likely to constrain migration patterns and lead to significant species losses during transition phases. Tropical forests are expected to warm by 2.0 °C above 1970s levels by the mid-21st century, with larger effects in continental interiors. However, changes in rainfall will be more important than changes in temperature. Where there are reductions in rainfall and higher temperatures, reductions in soil moisture are expected to be the most significant threat leading to significant dieback or changes in vegetation type in marginal areas. Interannual variability in large-scale climate events such as El Niño may act to exacerbate rainfall extremes. Depending on future climate scenarios, large losses (up to 30%) or expansions (up to 38%) of potential tropical forest area can be expected, but the impact of human activities, such as deforestation or fire, will be more important than climate change in determining forest cover in many tropical regions. Losses of forest cover in these biodiverse zones will cause significant species losses.

Tropical montane rain forests are likely to warm by 1.0-2.0 °C, but are most threatened by changes in the height of the cloud base. Clouds base height may rise by up to 2.0 m yr-1, forcing species dependent on high atmospheric moisture to migrate upwards. Where mountains are isolated and of lower altitude than the changed height of the cloud base, outward migration of species will be impeded and climate change will lead to complete losses of many species, including many endemics (those found nowhere else).

Mangrove forests will be able to adapt to rising temperatures, but may be threatened by rising sea levels. This threat will be particularly acute on sediment-poor coasts such as those found on small islands, and in areas where inland migration is constrained by human land use.

Forest management responses to climate change should focus on maintaining species diversity on national or continental scales through facilitating the processes of species migration, rather than by solely preserving specific reserves. Refugia and migration corridors may be best maintained by reducing habitat fragmentation and locating reserves near north-south running mountain ranges (boreal and temperate regions) or along precipitation gradients (tropical regions). Future forest management responses will need to incorporate the requirements of international agreements on the global economy and the global environment. The future implementation of the Kyoto Protocol is a key example of this and may lead to increased planting or forest conservation measures.