In December 2015 the delegates at the Paris Climate Conference concluded (again) that climate change poses a severe threat to the Earth. But how severe is the threat, really? In an attempt to set the record straight on this question this post summarizes the conclusions of the Intergovernmental Panel on Climate Change (IPCC), the organization responsible for communicating scientific data to policymakers at climate conferences so that they can make scientifically-informed policy decisions. Conclusions are quoted verbatim from the text of the 2013 IPCC Fifth Assessment Report (AR5). After reading them readers can make up their own minds whether it’s time to take to the hills.

First we need to set the ground rules. Delegates and speech-makers at climate conferences are not supposed to make up their own science. They are expected to rely on the scientific data provided to them by the IPCC:

One of the most important indicators of the policy relevance of the work of the IPCC is the use of its reports in international climate negotiations like the Conference of the Parties (COP) of the United Nations Framework Convention on Climate Change (UNFCCC). As stated on the UNFCCC website, the COP uses the information in IPCC reports as a baseline on the state of knowledge on climate change when making science based decisions.

And

…. science underpins the work of the Climate Change Convention …..

Next, sources of data. The IPCC consists of three Working Groups, only one of which (WG1) pronounces on the science. WG2 takes WG1’s conclusions and translates them into potential physical impacts and WG3 then takes WG2’s conclusions and makes recommendations as to what to do about them. In this post we consider only the WG1 results, i.e. the scientific conclusions.

Third, it’s important to recognize that the IPCC bases its future projections on four CO2 concentration scenarios for the 21st century. RCP (Representative Concentration Pathway) 2.6 is innocuous, RCP4.5 and RCP6 are middle-of-the-road and RCP8.5 is the worst-case. Unsurprisingly, the worst climate change impacts are projected to occur under RCP8.5 (Figure 1).

Figure 1. The IPCC’s four CO2 scenarios. Graphic from SkepticalScience

And which of these scenarios does the IPCC consider most likely?

all …. are considered possible depending on how much greenhouse gases are emitted in the years to come.

The IPCC also makes abundant use of terms such as “likely” and “very likely”. Figure 2 translates these terms into numerical probabilities. It also uses terms such as “high”, “medium” and “low” confidence. These are “expert judgement” assessments with no probability bounds placed on them:

Figure 2: The IPCC’s Likelihood Scale and associated probability of occurrence values. The boundaries are described as “fuzzy”.

Now to the summary. The quotes below are from the AR5 Working Group 1 Summary for Policymakers (SPM). The SPM begins with a summary of the observed effects of climate change, most of which are generally accepted as fact (the Earth’s surface has been warming overall, glaciers are receding overall, Arctic sea ice has been shrinking overall, sea levels show a net overall rise etc.) so we won’t bother to rehash them. The important conclusions – i.e. what the IPCC expects climate change to do in the future – are found later on under a series of “E” subheadings, and here’s a summary of what they say:

E. Future Global and Regional Climate Change

E.1 Atmosphere: Temperature

Increase of global mean surface temperatures for 2081–2100 relative to 1986–2005 is projected to likely be in the ranges 0.3°C to 1.7°C (RCP2.6), 1.1°C to 2.6°C (RCP4.5), 1.4°C to 3.1°C (RCP6.0), 2.6°C to 4.8°C (RCP8.5).

Figure 3: The IPCC’s global surface temperature projections for the 21st century (reproduced from Fig. SPM.7 (a))

E.2 Atmosphere: Water Cycle

Changes in the global water cycle in response to the warming over the 21st century will not be uniform. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions. The high latitudes and the equatorial Pacific Ocean are likely to experience an increase in annual mean precipitation by the end of this century under the RCP8.5 scenario. In many mid-latitude and subtropical dry regions, mean precipitation will likely decrease, while in many mid-latitude wet regions, mean precipitation will likely increase by the end of this century under the RCP8.5 scenario Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases.

E.3 Atmosphere: Air Quality

The range in projections of air quality (ozone and PM2.517 in near-surface air) is driven primarily by emissions (including CH4), rather than by physical climate change (medium confidence).

E.4 Ocean

Best estimates of ocean warming in the top one hundred meters are about 0.6°C (RCP2.6) to 2.0°C (RCP8.5), and about 0.3°C (RCP2.6) to 0.6°C (RCP8.5) at a depth of about 1000 m by the end of the 21st century. It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century. Best estimates and ranges for the reduction are 11% (1 to 24%) in RCP2.6 and 34% (12 to 54%) in RCP8.5. It is likely that there will be some decline in the AMOC by about 2050, but there may be some decades when the AMOC increases due to large natural internal variability. It is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century for the scenarios considered.

E.5 Cryosphere

Year-round reductions in Arctic sea ice extent are projected by the end of the 21st century. These reductions range from 43% for RCP2.6 to 94% for RCP8.5 in September and from 8% for RCP2.6 to 34% for RCP8.5 in February (medium confidence). In the Antarctic, a decrease in sea ice extent and volume is projected with low confidence for the end of the 21st century as global mean surface temperature rises. By the end of the 21st century, the global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6, and by 35 to 85% for RCP8.5 (medium confidence).

Figure 4: The IPCC’s Northern Hemisphere sea ice extent projections for the 21st century (reproduced from Fig. SPM.7 (b))

E.6 Sea Level

Global mean sea level rise for 2081–2100 relative to 1986–2005 will likely be in the ranges of 0.26 to 0.55 m for RCP2.6, 0.32 to 0.63 m for RCP4.5, 0.33 to 0.63 m for RCP6.0, and 0.45 to 0.82 m for RCP8.5 (medium confidence). For RCP8.5, the rise by the year 2100 is 0.52 to 0.98 m, with a rate during 2081 to 2100 of 8 to 16 mm yr–1 (medium confidence). Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. However, there is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.

Figure 5: The IPCC’s global sea level rise projections for the 21st century (reproduced from Fig. SPM.9)

E.7 Carbon and Other Biogeochemical Cycles

Earth System Models project a global increase in ocean acidification for all RCP scenarios. The corresponding decrease in surface ocean pH by the end of 21st century is in the range of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0, and 0.30 to 0.32 for RCP8.5.

Figure 6: The IPCC’s global ocean surface pH projections for the 21st century (reproduced from Fig. SPM.7 (c))

E.8 Climate Stabilization, Climate Change Commitment and Irreversibility

A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. Due to the long time scales of heat transfer from the ocean surface to depth, ocean warming will continue for centuries. Depending on the scenario, about 15 to 40% of emitted CO2 will remain in the atmosphere longer than 1,000 years. It is virtually certain that global mean sea level rise will continue beyond 2100, with sea level rise due to thermal expansion to continue for many centuries. There is high confidence that sustained warming greater than some threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a global mean sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C (low confidence) but less than about 4°C (medium confidence) global mean warming with respect to pre-industrial. Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic ice sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment.

The IPCC does not present any detailed projections for extreme weather events, which at this time seem to be causing more concern than anything else. Accordingly I have summarized the IPCC’s findings on observed trends in extreme weather events. These are reported in WG1 Chapter 2. Excerpts from the IPCC text are listed below:

Section 2.6. Changes in Extreme Events

Heatwaves (p212):

(T)here has been a likely increasing trend in the frequency of heatwaves since the middle of the 20th century in Europe and Australia and across much of Asia where there are sufficient data. However, confidence on a global scale is medium owing to lack of studies over Africa and South America but also in part owing to differences in trends depending on how heatwaves are defined. There is also evidence in some regions that periods prior to the 1950s had more heatwaves (e.g., over the USA, the decade of the 1930s stands out).

Heavy precipitation (p213):

(I)t is likely that since 1951 there have been statistically significant increases in the number of heavy precipitation events (e.g., above the 95th percentile) in more regions than there have been statistically significant decreases, but there are strong regional and sub-regional variations in the trends. In particular, many regions present statistically non-significant or negative trends.

Floods (p214):

(T)here continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale.

Droughts (p215):

(T)here is not enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought or dryness (lack of rainfall) since the middle of the 20th century. Based on updated studies, AR4 conclusions regarding global increasing trends in drought since the 1970s were probably overstated.

Small-scale severe weather (p216):

(T)here is low confidence in observed trends in small-scale severe weather phenomena such as hail and thunderstorms because of historical data inhomogeneities and inadequacies in monitoring systems.

Tropical storms (p217):

(I)ndices show upward trends in the North Atlantic and weaker upward trends in the western North Pacific since the late 1970s, but interpretation of longer-term trends is again constrained by data quality concerns. It is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin. Evidence, however, is for a virtually certain increase in the frequency and intensity of the strongest tropical cyclones since the 1970s in that region.

Extratropical cyclones (p220):

Studies continue to support a northward and eastward shift in the Atlantic cyclone activity during the last 60 years with both more frequent and more intense wintertime cyclones in the high-latitude Atlantic and fewer in the mid-latitude Atlantic. Some studies show an increase in intensity and number of extreme Atlantic cyclones while others show opposite trends in eastern Pacific and North America. Comparisons between studies are hampered because of the sensitivities in identification schemes and/or different definitions for extreme cyclones. In summary, confidence in large scale changes in the intensity of extreme extratropical cyclones since 1900 is low.

Next we come to the performance of the climate models on which the IPCC’s projections are based. The IPCC presents the following comparison of model output vs. observations in Fig TS9 (a) on p.60 of the IPCC Technical Summary:

Figure 7: IPCC model-observed global surface temperature comparisons. (CMIP3 & CMIP5 are different generations of climate models with CMIP5 the more recent. “Observations” show the HadCRUT4 combined land-ocean surface temperature series.)

And now I depart briefly from the IPCC data to plot a Figure of my own. Figure 8 compares CMIP5 multi-model output independently with the two temperature series that are combined to construct HadCRUT4. These are CRUTEM4, the surface air temperature series over land areas that contributes ~30% to HadCRUT4 and HadSST3, the sea surface temperature series over ocean areas that contributes the remaining ~70%. Readers can make up their own minds as to how good the fits are. The data are from KNMI Climate Explorer:

Figure 8: The Figure 7 model-observed comparisons shown separately for CRUTEM4 and HadSST3, the two series that are combined to produce HadCRUT4.

Finally we come to the question of the recent global warming “pause” or “hiatus”, the existence of which has recently been questioned. In the AR5 the IPCC acknowledges it as real (it’s clearly visible in Figures 7 and 8) and devotes a section (Chapter 9, Box 9.2) to discussing it:

Climate Models and the Hiatus in Global Mean Surface Warming of the Past 15 Years:

Almost all CMIP5 historical simulations do not reproduce the observed recent warming hiatus. There is medium confidence that the GMST trend difference between models and observations during 1998–2012 is to a substantial degree caused by internal variability, with possible contributions from forcing error and some CMIP5 models overestimating the response to increasing GHG and other anthropogenic forcing. In summary, the observed recent warming hiatus, defined as the reduction in GMST trend during 1998–2012 as compared to the trend during 1951–2012, is attributable in roughly equal measure to a cooling contribution from internal variability and a reduced trend in external forcing (expert judgment, medium confidence). The forcing trend reduction is primarily due to a negative forcing trend from both volcanic eruptions and the downward phase of the solar cycle. However, there is low confidence in quantifying the role of forcing trend in causing the hiatus, because of uncertainty in the magnitude of the volcanic forcing trend and low confidence in the aerosol forcing trend.

That concludes the summary of the IPCC’s scientific conclusions. They are, however, a little tedious, and for this reason a brief wrap-up is desirable. I’ll try and make it as objective and as closely in line with what the IPCC says as possible. During the 21st century, according to the IPCC:

Atmospheric CO2 concentrations will increase by somewhere between ~50ppm and ~500ppm. (Figure 1)

Global surface temperatures will increase by somewhere between 0.3°C and 4.8°C.

Arctic ice extent will decrease by somewhere between 8% and 94%.

Global glacier volume outside Antarctica will decrease by somewhere between 15% and 85%.

Sea levels will rise by somewhere between 0.26 m and 0.98 m

Ocean surface pH will decrease by between somewhere 0.06 and 0.32, lowering pH from its current levels of around 8.2 to the 7.8 – 8.0 range.

The Atlantic Meridional Overturning Circulation will weaken by somewhere between 1% and 54% but will not collapse.

Warming and sea level rise will continue after 2100 no matter what we do.

The Greenland ice sheet could eventually melt but it will take a thousand years or more. There are insufficient data to predict what might happen to the Antarctic ice sheet.

The climate models on which the above projections are based did not predict the recent warming hiatus.

Observations show no global trends in any extreme weather events.

So how serious is climate change, really? Are you going to take to the hills? Me, I think I’ll stay home and wait for more data.