It is 
virtually certain that human-caused CO2 emissions are the main driver 
of current global acidiﬁcation of the surface open ocean. {WGI SPM A.1, 
WGI SPM A.1.3, WGI SPM A.1.5, WGI SPM A.1.6, WG1 SPM A1.7, 
WGI SPM A.2, WG1.SPM A.4.2; SROCC SPM.A.1, SROCC SPM A.2}
Human-caused climate change is already affecting many weather and 
climate extremes in every region across the globe. Evidence of observed 
changes in extremes such as heatwaves, heavy precipitation, droughts, 
and tropical cyclones, and, in particular, their attribution to human 
inﬂuence, has strengthened since AR5 (Figure 2.3). It is virtually certain 
that hot extremes (including heatwaves) have become more frequent and 
more intense across most land regions since the 1950s (Figure 2.3), while cold 
extremes (including cold waves) have become less frequent and less severe, 
with high conﬁdence that human-caused climate change is the main 
driver of these changes. Marine heatwaves have approximately doubled 
74 
‘Main driver’ means responsible for more than 50% of the change. {WGI SPM footnote 12}
75 
See Annex I: Glossary.
in frequency since the 1980s (high conﬁdence), and human inﬂuence 
has very likely contributed to most of them since at least 2006. The 
frequency and intensity of heavy precipitation events have increased 
since the 1950s over most land areas for which observational data 
are sufﬁcient for trend analysis (high conﬁdence), and human-caused 
climate change is likely the main driver (Figure 2.3). Human-caused 
climate change has contributed to increases in agricultural and ecological 
droughts in some regions due to increased land evapotranspiration 
(medium conﬁdence) (Figure 2.3).