Climate Change 2007 Update

4. What impacts of climate change have already been observed?

In all regions of the world observations show that many natural systems are being affected by regional climate change, particularly by temperature increases.

Snow and ice are melting and frozen ground is thawing. The number of glacial lakes is increasing and so is ground instability in permafrost regions. The flow of rivers that are fed by melting snow and glaciers has increased, and the temperature of lakes and rivers has risen. Spring events such as migrations are starting earlier and the geographical spread of species is extending towards the poles. In addition, the CO₂ emitted by human activities has caused an increase of ocean acidity, with poorly understood but potentially important negative impacts.

Evidence accumulated over the past five years indicates that changes in many physical and biological systems are linked to the warming caused by human activities.

• Most of the warming observed since 1950 is very likely due to the increase of greenhouse gases generated by human activities.
• In most cases, when long term measurements revealed significant environmental change, the change went in the direction that would be expected as a result of a warming climate.
• Regions with significant changes are also the ones experiencing significant warming.
• Model projections have linked the changes in some physical and biological systems to the warming induced by human activities.

Despite limitations and gaps in knowledge, there is sufficient evidence to conclude with high confidence that over the last three decades the warming induced by human activities has had a measurable impact on many physical and biological systems.

Some impacts of regional climate change are only emerging at this stage. They remain difficult to discern because they also depend on factors other than climate or because some adaptation has taken place. Temperature increases could for instance already have affected agricultural and forest management, the number of heat related deaths, the spread of vectors of disease such as insects carrying malaria.

Recent changes are beginning to have effects, for instance mountain settlements are at risk of floods due to melting glaciers, the growing season in the Sahelian region of Africa is shortening, and damage from coastal flooding is increasing. More...