Climate Change Canada

 

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).