Executive summary
Physical and biological systems on all continents and in most oceans are already being affected by recent climate changes, particularly regional temperature increases (very high confidence) [1.3]. Climatic effects on human systems, although more difficult to discern due to adaptation and non-climatic drivers, are emerging (medium confidence) [1.3]. Global-scale assessment of observed changes shows that it is likely that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems [1.4].
Attribution of observed regional changes in natural and managed systems to anthropogenic climate change is complicated by the effects of natural climate variability and non-climate drivers (e.g., land-use change) [1.2]. Nevertheless, there have been several joint attribution studies that have linked responses in some physical and biological systems directly to anthropogenic climate change using climate, process and statistical models [1.4.2]. Furthermore, the consistency of observed significant changes in physical and biological systems and observed significant warming across the globe very likely cannot be explained entirely by natural variability or other confounding non-climate factors [1.4.2]. On the basis of this evidence, combined with the likely substantial anthropogenic warming over the past 50 years averaged over each continent except Antarctica (as described in the Working Group I Fourth Assessment Summary for Policymakers), it is likely that there is a discernible influence of anthropogenic warming on many physical and biological systems.
Climate change is strongly affecting many aspects of systems related to snow, ice and frozen ground (including permafrost) [1.3.1]; emerging evidence shows changes in hydrological systems, water resources [1.3.2], coastal zones [1.3.3] and oceans (high confidence) [1.3.4].
Effects due to changes in snow, ice and frozen ground (including permafrost) include ground instability in permafrost regions, a shorter travel season for vehicles over frozen roads in the Arctic, enlargement and increase of glacial lakes in mountain regions and destabilisation of moraines damming these lakes, changes in Arctic and Antarctic Peninsula flora and fauna including the sea-ice biomes and predators higher in the food chain, limitations on mountain sports in lower-elevation alpine areas, and changes in indigenous livelihoods in the Arctic (high confidence). [1.3.1]
The spring peak discharge is occurring earlier in rivers affected by snow melt, and there is evidence for enhanced glacial melt. Lakes and rivers around the world are warming, with effects on thermal structure and water quality (high confidence). [1.3.2]
The effects of sea-level rise, enhanced wave heights, and intensification of storms are found in some coastal regions – including those not modified by humans, e.g., polar areas and barrier beaches – mainly through coastal erosion [1.3.3.1]. Sea-level rise is contributing to losses of coastal wetlands and mangroves, and increased damage from coastal flooding in many areas, although human modification of coasts, such as increased construction in vulnerable zones, plays an important role too (medium confidence). [1.3.3.2]
The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic, with an average decrease in pH of 0.1 units. However, the effects of recent ocean acidification on the marine biosphere are as yet undocumented. [1.3.4]
More evidence from a wider range of species and communities in terrestrial ecosystems and substantial new evidence in marine and freshwater systems show that recent warming is strongly affecting natural biological systems (very high confidence). [1.3.5, 1.3.4]
The overwhelming majority of studies of regional climate effects on terrestrial species reveal consistent responses to warming trends, including poleward and elevational range shifts of flora and fauna. Responses of terrestrial species to warming across the Northern Hemisphere are well documented by changes in the timing of growth stages (i.e., phenological changes), especially the earlier onset of spring events, migration, and lengthening of the growing season. Changes in abundance of certain species, including limited evidence of a few local disappearances, and changes in community composition over the last few decades have been attributed to climate change (very high confidence). [1.3.5]
Many observed changes in phenology and distribution of marine species have been associated with rising water temperatures, as well as other climate-driven changes in salinity, oxygen levels, and circulation. For example, plankton has moved poleward by 10° latitude over a period of four decades in the North Atlantic. While there is increasing evidence for climate change impacts on coral reefs, separating the impacts of climate-related stresses from other stresses (e.g., over-fishing and pollution) is difficult. Warming of lakes and rivers is affecting abundance and productivity, community composition, phenology, distribution and migration of freshwater species (high confidence). [1.3.4]
Although responses to recent climate changes in human systems are difficult to identify due to multiple non-climate driving forces and the presence of adaptation, effects have been detected in forestry and a few agricultural systems [1.3.6]. Changes in several aspects of the human health system have been related to recent warming [1.3.7]. Adaptation to recent warming is beginning to be systematically documented (medium confidence) [1.3.9].
In comparison with other factors, recent warming has been of limited consequence in agriculture and forestry. A significant advance in phenology, however, has been observed for agriculture and forestry in large parts of the Northern Hemisphere, with limited responses in crop management. The lengthening of the growing season has contributed to an observed increase in forest productivity in many regions, while warmer and drier conditions are partly responsible for reduced forest productivity, increased forest fires and pests in North America and the Mediterranean Basin. Both agriculture and forestry have shown vulnerability to recent trends in heatwaves, droughts and floods (medium confidence). [1.3.6]
While there have been few studies of observed health effects related to recent warming, an increase in high temperature extremes has been associated with excess mortality in Europe, which has prompted adaptation measures. There is emerging evidence of changes in the distribution of some human disease vectors in parts of Europe. Earlier onset and increases in the seasonal production of allergenic pollen have occurred in mid- and high latitudes in the Northern Hemisphere (medium confidence). [1.3.7]
Changes in socio-economic activities and modes of human response to climate change, including warming, are just beginning to be systematically documented. In regions of snow, ice and frozen ground, responses by indigenous groups relate to changes in the migration patterns, health, and range of animals and plants on which they depend for their livelihood and cultural identity. Responses vary by community and are dictated by particular histories, perceptions of change and range, and the viability of options available to groups (medium confidence). [1.3.9]
While there is now significant evidence of observed changes in natural systems in every continent, including Antarctica, as well as from most oceans, the majority of studies come from mid- and high latitudes in the Northern Hemisphere. Documentation of observed changes in tropical regions and the Southern Hemisphere is sparse. [1.5]