Temperate forests have provided a net sink of carbon dioxide in recent decades due to regrowth of previously harvested regions and to increased productivity resulting from active management. Projections of future greenhouse gas emissions and atmospheric concentrations are highly uncertain and dependent on population and economic growth, sociological and biological factors, and new technologies. Most of the warming occurred prior to 1940 and before the greatest increase in human-derived greenhouse gas emissions. Although sea level rise, glacial retreat and other indirect indicators of a warming climate have been described as consistent with surface temperature trends, other factors raise questions about the measurements. Surface temperature measurement techniques have changed over time, are inequitably distributed across land and water surfaces, are biased upward by urbanization, and show more dramatic warming than do measurements made by satellites.
Some scientists also attribute a significant portion of warming to changes in solar output.
Future climate projections depend on many factors and assumptions. The validity of forest response projections is questionable because of these uncertainties in climate projections and in other factors such as disturbance, pests, and air pollution. Because assumptions and inclusions vary widely among models, regional projections can range from large-scale forest dieback to increased forest productivity. These variable projections are also susceptible to interpretation for management and policy based on other agendas. There is reason for optimism about the future of forests, however, since the most well documented factor that has changed, carbon dioxide, has generally positive effects on trees.