Uncertainties remain — particularly related to the timing, extent and regional variations of climate change — but there is mainstream scientific agreement on the following key facts:
- Carbon dioxide and other gases in the atmosphere create a greenhouse effect, trapping heat.
- Burning fossil fuels releases more carbon dioxide into the atmosphere.
- Carbon dioxide is the most significant greenhouse gas (not the most potent) because of the large quantities emitted.
- Carbon dioxide concentrations in the atmosphere are now the highest in 150,000 years.
- The 1990s was most likely the warmest decade in history, and 1998 the warmest year.
- Warming of about 1.3 C, compared to pre-industrial levels, is probably inevitable because of greenhouse emissions so far.
- Scientists say it is vital that warming be limited to less than 2 C to prevent the worst effects of climate change.
- If greenhouse gas emissions are not brought under control, the speed of climate change over the next 100 years will be faster than before the dawn of civilization.
- There is a possibility that climate feedback mechanisms will result in a sudden and irreversible climate shift.
There is a broad scientific consensus that humanity is in large part responsible for climate change, which will likely have disastrous consequences of it isn’t addressed. According to statement issued in 2005 by the national science academies of the United States, United Kingdom, France, Russia, Germany, Japan, Italy, Canada, Brazil, China and India:
“There will always be uncertainty in understanding a system as complex as the world’s climate. However, there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001). This warming has already led to changes in the Earth’s climate.
“We urge all nations, in line with the principles of the United Nations Framework on the Convention on Climate Change (UNFCCC), to take prompt action to reduce the causes of climate change, adapt to its impacts and ensure that the issue is included in all relevant national and international strategies.”
The Intergovernmental Panel on Climate Change (IPCC) says:
- Most of the observed warming over the past 50 years is likely due to the increase in greenhouse gas concentrations.
- There is new and stronger evidence that most of the observed warming over the past 50 years is attributable to human activities.
- About three-quarters of the human-caused emissions of CO2 to the atmosphere during the past 20 years are due to fossil fuel burning.
The IPCC assessment of climate change goes on to warn that there is a risk of feedback loops, which could cause runaway climate change, and that the global warming to date is already having an effect on the biosphere.