Signs, Effects, Causes

Models of 20th century climate run with and without anthropogenic greenhouse gases

Though the greenhouse gas hypothesis has been thoroughly poked and prodded, no credible competing explanation has emerged that fits the bulk of the evidence. If greenhouse gases had remained unchanged, scientists found that solar and volcanic activity during the last half century should have produced cooling, not warming.  Thus, the IPCC concluded it is "very likely", or greater than a 90% chance, that something other than natural causes is at work.

Here are the IPCC's confidence levels in other signs and effects of climate change, as well as for the evidence that humans are responsible.

Signs Content

IPCC Confidence in Signs of Climate Change
Signs Certainty % Probability
Warming of the climate system Unequivocal 100%
Less frequent cold days, cold nights, and frosts, more frequent hot days and hot nights over most land areas Very likely >90%
During the second half of the 20th century, highest average Northern Hemisphere temperatures in at least the past 1,300 years Likely >66%
Global area affected by drought has increased since the 1970s Likely >66%

Observed Effects Content

IPCC Confidence in Observed Effects of Climate Change
Effects Confidence Level Quantitative Confidence
Recent warming strongly affects terrestrial biological systems, including earlier spring events and poleward and upward shifts in plant and animal ranges Very high At least 9 out of 10 chance of being correct
Increases in number and sizes of glacial lakes High About 8 out of 10 chance of being correct
Increasing ground instability in mountain and other permafrost regions High About  8 out of 10 chance of being correct
Increased runoff and earlier spring peak discharge in many glacier and snow-fed rivers High About 8 out of 10 chance of being correct
Warming of lakes and rivers in many regions, with effects on thermal structure and water quality High About 8 out of 10 chance of being correct

Human Attribution Content

IPCC Confidence in Human Attribution
Evidence humans are responsible Certainty % Probability or Quantitative Confidence
Global average net effect of human activities since 1750 is warming Very high confidence At least 9 out of 10 chance of being correct
Solar + volcanic forcings have produced cooling in last 50 years Likely >66%
Most of observed increase in global average temperatures since mid-20th century due to increases in greenhouse gases from human activities Very likely >90%
Human-caused warming over last 30 years has a had a visible influence on many physical and biological systems Likely >66%