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 20 th 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%

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

Evidence humans are responsible

Certainty

% Probability or Quantitative Confidence

Global average net effect of human activities since 1750 one of 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%

Effect

IPCC Likelihood

% Probability, Confidence

20-30% of plant and animal species become extinct if global average temperatures exceed 2.7-4.5°F (1.5-2.5°C)

Likely (extinction rate)
Medium confidence if temperatures exceed 2.7-4.5°F (1.5-2.5°C)

>66% (extinction rate),
5 out of 10 chance extinction will occur if temperatures exceed 2.7-4.5°F (1.5-2.5°C)

By 2080, millions experience floods due to sea-level rise

Very high confidence

At least 9 out of 10 chance

Frequency of warm spells and heat waves increases

Very Likely

>90%

Frequency of heavy precipitation increases

Very likely

>90%

Areas affected by drought increase

Likely

>66%

Intense tropical cyclone activity increases

Likely

>66%

Millions experience adverse health effects, especially in undeveloped countries

Likely

>66%