[Update: several clarifications have been added in the best case scenario section.]

The complete, 2500 pages long Working Group One (WG1) report of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) has been published. While the devil is often in the details buried deep in those 2500 pages, the Summary for Policymakers (SPM) is a distillation of the key scientific findings that the WG1 authors and every national government agree upon. As such, the SPM is an inherently conservative¹ summary of the science.

Which makes the the SPM’s findings that climate is changing, that human activity is extremely likely to be the dominant cause of those changes, and that the changes, both individually and in combination, are very likely to be disruptive to natural ecosystems, human society, or both all that much more impressive.

“Warming of the climate system is unequivocal”

According to the SPM, there is no longer any question about whether the Earth has warmed. Not only that, but the SPM also declares that many of the observed changes are “unprecedented” over periods ranging from decades to millennia.

First, the SPM says that “solar irradiance changes and stratospheric volcanic aerosols made only a small contribution to the net radiative forcing throughout the last century, except for brief periods after large volcanic eruptions.” Radiative forcing is a term used in part to describe how much energy is retained in the Earth’s climate system (oceans, atmosphere, and ecosystems) due to a particular effect. In this case, the SPM is saying that science has ruled out changes in the intensity of sunlight and volcanic gasses as significant contributors to the “unequivocal” warming.

Second, according to the SPM the Medieval Climate Anomaly (aka the Medieval Warm Period) was just as warm in some regions between 950 and 1250 as the late 20th century has been. However, the SPM points out that those warm periods were not “as coherent across regions as the warming in the late 20th century.” This means that whatever warming occurred during the Medieval Climate Anomaly affected certain parts of the globe during one period of time (950 to 1150), other parts of the Earth during a second period (1125 to 1200), and so on. By comparison, the modern warming has been nearly global in extent and occurring at the same time in nearly all regions.

Third, the SPM says that the concentrations of the three most potent greenhouse gases – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – “substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years.” But more important is the SPM’s “very high confidence” that the rate of increase in the concentrations is “unprecedented in the last 22,000 years.” This means that the last time greenhouse gas concentrations might have risen this fast was at the end of the last ice age.

Human influence has been the dominant driver of climate disruption

The SPM writes “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. (emphasis original)” That emphasized bit is critical, because according to the strict rules under which the SPM operates it means that human activity has between a 95% and 100% chance of being the dominant cause of climate warming. Even more important is that this is not just an agreement among the WG1 authors, but among the international community as well. The language of “extremely likely” was reviewed and approved by every nation involved in the IPCC, even those nations like Saudi Arabia and Russia who, in 2007 during the Fourth Assessment Report, rejected such unequivocal language since it might threaten their national interests.

According to the SPM, human influence has been felt throughout the climate system:

• Tropospheric warming and corresponding lower stratospheric cooling: very likely (90%+ likelihood)
• Increasing global upper ocean (0-700 m depth) heat content: very likely (90%+ likelihood)
• Increasing number of hot daily record temperatures and decreasing number of cold daily record temperatures: very likely (90%+ likelihood)
• Increasing moisture in the atmosphere: likely (66%+ likelihood)
• More intense rain and snowfall: likely (66%+ likelihood)
• More than doubled the probability of heat waves in some parts of the world: likely (66%+ likelihood)

And perhaps the most impressive influence identified in the SPM is in Arctic sea ice extent. The SPM asserts, with medium confidence, that both the summer sea ice retreat and Arctic ocean sea surface temperatures are “unprecedented… in at least the last 1,450 years” and attributes that retreat to human influence.

The “best-case” scenario

There are essentially two key points that the SPM makes about it’s projections for the rest of this century. First, the more greenhouse gases humanity emits, the worse things will get. And second, it will take a “substantial and sustained” effort to reduce greenhouse gas emissions to avoid the most serious projected consequences.

The SPM uses “Representative Concentration Pathways” (RCPs) to standardize models such that they can be compared. Each RCP represents a plausible future with the number identifying how much extra energy is being forced into the Earth’s climate system by greenhouse gases by the end of the century. RCP2.6 represents a “best case” scenario where greenhouse gas emissions peak at around the middle of the century and then drop somewhat by 2100. RCP4.5 and RCP6.0 both stabilize greenhouse gas emissions shortly after 2100 and are supposed to represent intermediate cases where the international community works to stabilize emissions but is unable to do so quickly. RCP8.5 is a plausible “worst-case” scenario where no-one even tries to stabilize greenhouse gas emissions, and they continue to increase after 2100.

[For a reasonably detailed introduction to what RPCs are and why they’re used in AR5, please read the Skeptical Science Beginner’s Guide to Representative Concentration Pathways.]

In order to meet the “best case” RCP2.6 scenario, the maximum amount of CO₂ that humanity can emit and still fall within the scenario’s parameters is between 140 and 410 GtC (gigatonnes of carbon) from energy, cement, industry, and waste. For comparison, from 2007 to 2011 the US Energy Information Administration estimates that the total global emissions of carbon dioxide from energy production was approximately 42 GtC. Assuming the emission rate stays constant, we have between 17 and 50 years before we’re committed to exceeding the best-case scenario. In reality we have fewer years because global economic growth drives increasing energy use, especially in the developing world.

The EIA’s estimates also fail to include non-energy related emissions such as land use changes due to agriculture. And the SPM also points out that adding other greenhouse gases like methane and nitrous oxide, or improving air quality by reducing aerosol pollution, results in a reduction in the maximum emission threshold and a corresponding lower number of years before we exceed the “best case” threshold.

This is what the SPM says that the “best-case” scenario will look like:

• Global mean surface temperature increases 0.3 to 1.7 °C (0.5 to 3.1 °F) relative to the 1985-2005 mean temperature. (likely, 66%+)
• More common hot extremes and less common cold extremes (virtually certain, 99%+)
• Dry areas get drier and wet areas get wetter (likely, 66%+)
• Monsoon winds will decrease while monsoon precipitation and duration increase (likely, 66%+)
• Extreme precipitation (rains leading to flooding, blizzards, et al) will be more common in wet tropical and mid-latitudes (very likely, 95%+)
• The Earth’s oceans will heat up about 0.6 °C (1.1 °F) in the top 100 m and about 0.3 °C (0.5 °F) at depths of 1000 m.
• Arctic summer sea ice extent will drop an average of 43% (medium confidence).
• The total volume of all glaciers (excluding Antarctica) will drop between 15 and 55% (medium confidence).
• The area of permafrost in the top 3.5 m of frozen ground is projected to decrease an average of 37% (virtually certain, 99%+)
• Sea level will rise between 0.26 and 0.55 m (~10 to 22 inches) by 2100 (likely, 66%+
• Ocean acidification will continue, decreasing the pH of the ocean surface by 0.06 or 0.07 units (a 14-17% increase in hydrogen ions)

Finally, it’s worth nothing that the SPM says that achieving the “best case” RCP2.6 projections requires reducing CO₂ emissions to zero or even actively removing CO₂ from the atmosphere via some form of geoengineering by the year 2100. Both would require effective national and international political institutions and a wholesale restructuring of how humanity generates and uses energy.

The recent slowing of surface temperature warming

There has been a lot of discussion in the media about how over the last 15 years the rate of surface temperature warming has slowed, with some media outlets referring to this as a “pause” or a “hiatus.” The SPM addresses these claims directly, saying that the change is due to two factors – a reduction in the amount of energy being absorbed by the Earth’s climate and a change in how the energy is distributed. The SPM attributes the energy reduction in part to the fact that the solar cycle went from maximum to minimum between 2000 and 2008 and in part to volcanic aerosols. The SPM attributes the redistribution of energy to ocean absorption, especially the El Nino Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the absorption of energy by the deep ocean (700 – 2000 m deep).

The SPM also briefly addresses why there was a difference between the reduced surface warming and model outputs. The SPM states that models “are not expected to reproduce the timing of internal variability” such as ENSO even as the document admitted that some models might “overestimate the response to increasing greenhouse gases… (dominated by the effects of aerosols).”

In essence, the SPM says that other changes in the climate system – the continued decline in Arctic summer sea ice extent, the continued rise in global mean sea level, and the fact that the 2000 to 2010 decade was the hottest decade in the instrumental record – demonstrate that the observed reduction in surface warming was weather, not climate. And the SPM essentially said that that demanding climate models predict the weather was not what the models were designed for.

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The SPM finds that the Earth’s climate has warmed unequivocally and that many of the changes have been unprecedented over periods from decades to millennia. In addition, the “best-case” scenario presented in the SPM implies both major changes to how humanity generates and uses energy and to the effectiveness of social and political institutions. And the SPM concludes that much of the recent focus on the reduced rate of surface warming was misplaced.

As a consensus document, the SPM is inherently conservative. Unanimous approval by IPCC member nations is a difficult threshold to reach, which makes the following statement from the SPM just that much more impressive:

Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely [95-100% likely] that human influence has been the dominant cause of the observed warming since the mid-20th century. (emphasis original)

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Footnotes:
1: Used in this way, “conservative” means “marked by moderation or caution.” It has no connection to either religious practice or political philosophy.