A February report by the International Panel on Climate Change states that "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level."
While such large-scale changes to our planet cannot be easily detected by the casual observer, they can spot unusual weather and changes occurring in their local environment.
From December until March, the BBC News website solicited comments from around the world to find out how climate change has affected people where they live.
Molly Rossmiller sent this picture of daffodils in early January in the UK
They then asked the Institute for Global Environmental Strategies, near Washington DC, to sort and summarize the responses, and place them in context with scientific data and climate change research.
Though many responders offered their political and scientific views on climate change and the role of human activity, the primary goal of this analysis was to review claims about how climate change may be manifesting itself in people's everyday lives, regardless of whether the cause is natural or manmade.
The responses received suggest that people across the globe are connecting a variety of locally observed changes in weather and the environment to climate change.
In some cases scientific data and research support such a link, while in others the evidence is not sufficient to identify climate change as a direct cause.
A large majority of comments from Europe, especially the UK, and North America focus on warmer winters, less snow, melting ice and shrinking glaciers, and resulting changes in wildlife and vegetation, all of which are widely believed by scientists to be influenced by climate change.
To a lesser extent, responders mention changes in year-round weather patterns and the occurrence of more extreme temperature and precipitation events, though studies show the impact of climate change on these phenomena is not as certain.
Of particular interest to many Europeans and North Americans was the unusual warmth of this past winter, which according to numerous comments led to the early blooming of flowers and fruits, and the appearance of butterflies, bees, mosquitoes and other insects in the middle of winter.
While most scientists assess climate change and its impacts over the course of multiple years and decades rather than single years or seasons, the Met Office has described this past winter, the second warmest on record for the UK, as consistent with conditions expected from a changing climate.
Similarly, the U.S. National Oceanic and Atmospheric Administration identified the long-term warming trend as a contributing factor to an unusually warm 2006, the warmest year on record in the US.
Early sightings of plant and animal life are supported by data from Springwatch, an Internet-based project that engages the public in tracking the first signs of UK spring.
Geoff McCormack spotted this Red Admiral in January
According to Springwatch, which is run by the BBC in association with the Woodland Trust and the UK Phenology Network, peacock butterflies began appearing by late December and red-tailed bumblebees by the middle of February.
Springwatch says that some bumblebee species are found further north than they were 30 years ago, with some appearing so early in the season they have time to raise an extra generation.
Regarding mosquitoes, various studies show that a warming climate is expanding their geographic range and could be contributing to the spread of diseases such as malaria and West Nile Virus.
A 2001 study by biologists at the University of Oregon found that mosquitoes have actually altered their genes in response to warming, enabling more of them to survive the winter.
Disappearing snow and ice
Americans and Canadians report warmer winters with less snow.
While true for many locations, parts of the western US and western New York received near-record snowfalls this past winter. As of March 1, some locations in the western US had more than 100 percent of their normal snowpack.
This harbour in Greenland is usually frozen over
A number of commenters, from North America and Europe, claim that lakes and rivers do not freeze for as long as they used to. Confirming this, a 2000 study by an international team of scientists found that lakes and rivers in the Northern Hemisphere freeze later and thaw earlier than they did in the 1850s.
This warming of waterways is consistent with what a Pennsylvania farmer describes as a decrease in trout, a cold-water fish, and an increase in carp, a warm-water fish.
Meanwhile, comments from mountaineers and ice climbers blame disappearing ice in areas such as the French Alps on longer warm spells. The relationship between climate warming and glacier melt has been well documented, with scientists at the University of Zurich in Switzerland predicting that rising temperatures could melt from 80 to 100 percent of Alpine glaciers by 2100.
Too Dry, Too Wet
Responders from parts of England, the western US, Africa and Australia tell stories of decreasing rainfall and increasing drought. This is especially true of comments from Australia where dry weather has triggered water restrictions.
A study by Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) finds that drought in Australia is largely the result of natural weather patterns, but may be increasingly enhanced and made more frequent by climate change.
CSIRO's analysis is consistent with research on the connection between drought and climate change for other regions of the world.
Commenters from New Zealand and parts of Asia write not about drought, but increased rainfall and flooding. Various studies suggest that warmer ocean temperatures and shifts in rainfall patterns due to climate change may already be contributing to more extreme rainfall, though it is difficult to link specific events, such as the historic flooding seen in Malaysia in December and January, to climate change.
Strange and stormy
Icy temperatures are not usual in Texas, USA says Emilio Jimenez
Capturing the attention of residents along the US and Canadian west coast is stormy and strange weather, such as this past winter's unusual cold snap in California, which included a rare snowfall in parts of Los Angeles, and a windstorm in British Columbia that fell more than 3,000 trees.
However, most scientists agree that individual storms, cold blasts or heat waves cannot be directly tied to climate change.
In fact, one recent study blames pollution from Asian factories, not a warming climate, for altering the storm track across the Pacific Ocean and intensifying storms.
Linking weird weather to climate change is not completely without merit, though, as studies from both the US National Center for Atmospheric Research and the University of Washington, US, predict that a changing climate is likely to result in more weather extremes.