2.3.4: Detection and attribution of
observed changes
Establishing
the causes of observed changes in climate involves both “detection” and
“attribution.” Specifically, “detection” means demonstrating that an observed
change is inconsistent with internal climate variability; in effect, this is a
task of detecting a signal from the “noise” of background climate variability.
“Attribution” means identifying the causes of an observed change in terms of
different forcings (Bindoff et al., 2013). The IPCC AR5 included a chapter
(Bindoff et al., 2013) assessing the evidence for attributing global and
regional changes in a range of variables to GHG increases and other forcings.
Understanding the causes of climate change on the global scale is important for
understanding the causes of regional climate change discussed in Chapters 4 to
7 of this report. In this subsection, we summarize relevant findings from the
IPCC AR5 assessment and more recent findings on global-scale attribution. The
relatively new science of attribution of individual events, as opposed to
longer-term changes, is discussed.
Detection
and attribution studies compare observed climate changes with simulations from
different types of climate-model experiments: 1) simulations of the response to
external forcings of interest; and 2) simulations with no variations in
external forcing that show the effect of internal climate variability.
Confidence in such analyses is increased by using simulations from multiple
climate models developed in centres around the world, and by validating
simulated internal variability by comparison with observations. If an observed
change is inconsistent with simulated internal variability alone, then a
response to external forcing is detected. If the observed change is consistent
with model simulations including a particular forcing, such as GHGs, and
inconsistent with simulations omitting it, then the observed change is
attributed, in part, to that forcing. Since more than one forcing drives trends
in climate, an observed change is generally not wholly attributable to
variations in one forcing. The sections below summarize attribution of observed
changes in each component of the climate system.
Atmosphere and surface
The IPCC AR5
assessed contributions of greenhouse gases, other anthropogenic forcings
(mainly aerosols), and natural forcings to the observed trend in GMST that
increased approximately 0.6ºC from 1951 to 2010, based on several studies that
had assessed these trends quantitatively using detection and attribution
methods. The trend attributable to combined forcings from human activities
(mainly changes in GHGs and aerosols) is likely between 0.6ºC and 0.8ºC (see
Figure 2.10) and extremely likely more than half of the observed increase
(Bindoff et al., 2013). Note that, as expected, IPCC AR5 assigned a lower
likelihood level to the narrower confidence interval (0.6ºC to 0.8ºC) and a
higher likelihood level to the broader one (greater than half the observed
warming). However, when the GMST response to forcings is separated into contributions
from GHG forcing and aerosol forcing, uncertainties are larger due to several
factors: large uncertainties in aerosol forcing, differences in the simulated
responses to these forcings among models, and difficulties in separating the
response to GHG increases from the response to aerosol changes. Nonetheless,
more than half of the observed increase in GMST was very likely due to the
observed human-caused increase in GHG concentrations. The combined effect of
aerosols from volcanic eruptions and variations in solar irradiance made only a
small contribution to observed trends over this period (statistically, the
contribution was not significantly different from zero). Similarly, internal
variability made only a small contribution to trends over this period. Warming
was also observed over the first half of the 20th century, and this warming was
very unlikely to have been due to internal variability alone, but it remains
difficult to quantify the contributions of internal variability, anthropogenic
forcing, and natural forcing to this warming (Bindoff et al., 2013).
Since the
publication of the IPCC AR5, studies have shed further light on aspects of
detection and attribution. For example, the influence of observational
uncertainty on estimates of the GMST trend attributable to GHGs was found to be
small relative to other sources of uncertainty (Jones and Kennedy, 2017).
Another study found that considerable differences remain among models in the
simulated response to forcings from human activity, particularly to non-GHG
forcing (Jones et al., 2016). However, the conclusions of these studies remain
consistent with the IPCC AR5 (Bindoff et al., 2013). Even when using a novel
approach to detection and attribution (Ribes et al., 2017), the assessed range
for the contribution to observed warming trends from human activities remains
consistent with the range in the IPCC AR5 (Bindoff et al., 2013).
The IPCC AR5 also assessed that it was likely that forcings from human activity have contributed to warming of the lower atmosphere (troposphere) since 1961 (Bindoff et al., 2013). Recent research continues to support this assessment. A new study found that apparent differences between the rate of warming of the lower atmosphere in climate models and in satellite observations since 1979 are smaller than previously reported (Santer et al., 2017).
There is medium confidence that human activities have contributed to observed increases in atmospheric specific humidity and to global-scale changes in precipitation over land since 1950, including increases in the Northern Hemisphere at mid- to high latitudes (Bindoff et al., 2013). Large uncertainties in observations and models, and large internal variability in precipitation, precluded greater confidence. Research since the IPCC AR5 (e.g., Hegerl et al., 2015; Polson et al., 2016) has examined sources of uncertainty in more detail, but the overall conclusions remain consistent with those of IPCC AR5 (Bindoff et al., 2013).
Ocean
Several
aspects of observed global-scale change in the oceans have been attributed to
human activity. In particular, it is very likely that human-caused forcing made
a substantial contribution to upper ocean warming since 1970 and to a rise in
global mean sea level since the 1970s (Bindoff et al., 2013). It is very likely
that human-caused increases in CO2 have driven acidification of ocean surface
waters, through uptake of CO2 from the atmosphere, decreasing pH by 0.0014 to
0.0024 per year (see Chapter 7, Section 7.6.1). Recent research continues to
support the attribution of ocean warming and sea level rise to human influence
(e.g., Slangen et al., 2014; Weller et al., 2016), with new estimates of the
heat content of the upper ocean showing a larger warming trend than that
assessed in the IPCC AR5 (Durack et al., 2014).
Cryosphere
It is very
likely that human-caused forcings have contributed to Arctic sea ice loss since
1979 (Bindoff et al., 2013). This conclusion was based on model simulations,
which were able to reproduce the observed decline only when including
human-caused forcings. There is low confidence in the understanding of an
observed increase in the extent of Antarctic sea ice. However, since that
assessment was made in 2013, Antarctic sea ice extent has decreased, with
September 2017 sea ice extent being the second lowest on record (NOAA, 2017).
It is likely that human-caused forcing contributed to the observed surface
melting of the Greenland Ice Sheet since 1993 and to the observed retreat of
glaciers since the 1960s, but there is low confidence in attribution of the
causes of mass loss from the Antarctic Ice Sheet. There was likely a
contribution of human activity to observed reductions in Northern Hemisphere
snow cover since 1970 (Bindoff et al., 2013). New research strengthens the
evidence for attribution of the decrease in Arctic sea ice extent (e.g.,
Kirchmeier-Young et al., 2017), and Northern Hemisphere snow cover (e.g.,
Najafi et al., 2016) to human influence.
Extremes
On the
global scale, it is very likely that human-caused forcing has contributed to
observed changes in the frequency of daily temperature extremes since 1950,
including increases in hot extremes and decreases in cold extremes (Bindoff et
al., 2013). For regions with sufficient observations, there is medium
confidence that human-caused forcing has contributed to increased intensity of
heavy precipitation events since 1950. New research further strengthens the
evidence for attribution of changes in temperature and precipitation extremes
to human influence (Zhang et al., 2013; Kim et al., 2016; Fischer and Knutti,
2015; Christidis and Stott, 2016).
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