Last week, the Intergovernmental Panel on Climate Change (IPCC) released the first (and primary) instalment of its Fifth Assessment Report (AR5). Its key conclusions include:
- The Earth is accumulating heat.
- Heat-trapping greenhouse gases are rising: Atmospheric levels of carbon dioxide (CO2), methane, and nitrous oxide have reached levels unprecedented in at least 800,000 years because of human activity. Since 1750, CO2 has increased 40% to 391 parts per million (ppm), primarily from fossil fuel emissions. Global fossil fuel burning in 2011 emitted 9.5 gigatonnes of carbon (GtC) or 34.8 Gt CO2.
- 29% of the emitted CO2 has gone into the oceans, causing a 26% increase in ocean acidity. (28% of the CO2 has been absorbed by land vegetation.)
- The human-caused change to the Earth’s energy balance since 1750 is 2.3 watts per square metre (W/m2), and the main cause is increasing CO2 (1.8 W/m2).
- The warming effect of greenhouse gases (3 W/m2) has been partially offset by the temporary cooling effect of reflective particulate pollutants called aerosols (-0.9 W/m2). Natural influences such as solar and volcanic activity have been negligible relative to 1750.
- The Earth’s climate is warming, and observed changes since the 1950s have been unprecedented over decades to millennia.
- The atmosphere is warming: The global average surface temperature has risen 0.8°C since 1850, and almost the entire surface has warmed. Each of the last three decades has been the warmest on record, though the rate of warming varies over short periods. Hot extremes have increased and cold extremes have decreased. The Northern Hemisphere is likely at its warmest temperature in 1,400 years, though some regions reached similar warmth in medieval times. Warming has been greater over land than ocean. The Arctic has warmed very rapidly.
- The ocean is warming: Most of the added heat is accumulating in the oceans, causing the ocean temperature to increase in recent decades. Though upper-ocean warming appears to have slowed since 2003, the deep ocean has continued to warm.
- Ice is melting at an accelerating rate: Arctic sea ice is disappearing to an extent unprecedented in at least 1,450 years. The Greenland and Antarctic ice sheets are losing mass several times faster than in the 1990s. Glaciers are retreating, permafrost is shrinking, and Northern Hemisphere spring snow cover is decreasing (though sea ice has increased in some Antarctic regions for reasons not well understood).
- Global sea level is rising: Although the sea has been slowly rising for millennia, the rise has accelerated through the 19th, 20th, and 21st centuries. The rise is half due to ocean thermal expansion and half from land ice loss.
- Climate models are able to explain observed climate change over decades and in each continent and ocean. The slower warming trend of the last 15 years (which falls within the range of model projections) is attributed partly to redistribution of heat in the oceans, and partly to the solar cycle and possibly to other poorly quantified influences.
- It is >95% likely that humans are the dominant cause of observed warming since the mid-20th century.
- Natural influences may have had a net cooling effect in recent decades.
- Human influence on the climate system is detectable in atmospheric warming, ocean heating, the changing global water cycle, melting ice, rising sea level, and increasing extreme weather.
- The sensitivity of the climate system to changes in CO2 is in the range of 1.5-4.5°C per doubling of atmospheric CO2, with >66% likelihood, based on observations and models. It is <5% likely to be <1°C and <10% likely to be >6°C.
- A further 0.3-0.7°C of global warming relative to 1986-2005 is likely by 2035 (assuming no major natural climate events).
- The rate of warming will continue to vary chaotically between years, decades, and regions.
- Continued greenhouse gas emissions will cause further global warming and climate change.
- The RCP2.6 emissions scenario, where the atmospheric CO2 concentration peaks at 420 ppm (and CO2e at 480 ppm) and then declines, is the only possible future in which global warming is <33% likely to exceed 2°C relative to preindustrial temperature by 2100. It is also the only scenario in which warming will not continue beyond 2100.
- In the RCP4.5 scenario, where CO2 is stabilized at 540 ppm (CO2e 630 ppm), global warming is >66% likely to exceed 1.5°C by 2100 and >50% likely to exceed 2°C by 2100, but <33% likely to exceed 4°C by 2100.
- In the RCP6.0 scenario, where CO2 rises to 670 ppm (CO2e 800 ppm) and stabilizes after 2100, global warming is >66% likely to exceed 2°C by 2100, but <33% likely to exceed 4°C by 2100.
- In the RCP8.5 scenario, where CO2 rises to 940 ppm (CO2e 1,300 ppm) and continues rising after 2100, global warming is >66% likely to exceed 2°C by 2100, and ~50% likely to exceed 4°C by 2100.
- The land will continue to warm faster than the ocean, and the Arctic faster than the rest of the globe.
- Hot extremes will continue to get more frequent, and cold extremes less frequent.
- Wet regions and periods are projected to get wetter, while dry regions and periods get drier (with regional variation). Intense rainfall events will become more intense.
- The ocean will continue to warm as heat penetrates to the deep layers.
- Arctic sea ice will continue to shrink and thin, though models cannot predict with confidence when the Arctic Ocean will become nearly ice-free in September.
- Glacier volume, snow cover, and permafrost will continue to decrease.
- Sea level will continue to rise throughout the 21st century and beyond. Regardless of emissions, the rate will exceed that observed since 1970, due to increasing ocean warming and loss of land ice. Sea level could rise up to one metre by 2100, with some evidence for up to two metres, though the rise will not be uniform across all regions.
- The threshold for collapse of the Greenland ice sheet (which would eventually raise sea level by up to 7 m) is estimated to be between 1-4°C. The threshold for large Antarctic ice loss is unknown.
- Ocean acidification will increase.
- Global warming is expected to cause carbon cycle feedbacks within the 21st century, which will add to CO2 levels and amplify global warming. In RCP8.5, 50-250 GtC could be released from melting permafrost. However, the oceans are expected to continue absorbing CO2 through to 2100.
- Limiting climate change requires substantial and sustained emissions reductions.
- Total cumulative CO2 emissions will be the main factor in the magnitude of future global warming.
- To have a 66% chance of avoiding 2°C, humanity must limit future CO2 emissions to less than 270 GtC (about 28 years of global emissions at the current rate), with a minimum estimate of 140 GtC (about 15 years).
- Most aspects of climate change will persist for centuries even if CO2 emissions are stopped. Much climate change is irreversible unless there is sustained large net removal of CO2 from the atmosphere.
- Proposed geoengineering methods, both for carbon dioxide removal and solar radiation management, have limitations, carry side-effects, and/or lack permanence. There is not enough evidence to comprehensively assess such approaches.
Deniers have seized upon the slightly lower minimum estimate of climate sensitivity and the slow warming rate of the last 15 years, claiming these are admissions that the IPCC’s previous reports were wrong. In reality, these are relatively minor details, and the big picture emerging from AR5 is that human-caused global warming and its impacts are more certain and more serious than ever.
Deniers paint the IPCC as alarmist. In reality, the IPCC’s conclusions are very conservative due to the cumbersome, bureaucratic, time-consuming, consensus-based process through which its reports are produced. Judging from the summary for policymakers, potential shortcomings of AR5 include:
- It glosses over observational evidence of fast-approaching tipping points.
- Its projections for Arctic sea ice melt (from simulations published a few years ago) have already been exceeded by real-world observations.
- It refers to the shrinking extent of Arctic sea ice, but ignores the 80% reduction in its volume since 1979, which suggests it could be all gone within a few years.
- It fails to explain that an ice-free Arctic Ocean, by reversing the surface reflectivity of the northern polar region, threatens to set off a chain reaction of tipping points, including large-scale release of carbon from melting permafrost and collapse of the Greenland ice sheet.
- It neglects to mention the recent alarming observation that permafrost is starting to thaw and emit carbon.
- It downplays the possibility that the observed exponential acceleration of ice sheet mass loss could continue.
- Its estimates of climate sensitivity (and thereby projections for global warming and sea level rise) ignore evidence from the paleoclimate record indicating that even the present CO2 level and global temperature could set off large unmodelled amplifying feedbacks; and that a doubling of CO2 is associated with long-term warming much higher than 3°C and long-term sea level rise of tens of metres.
- Its recommendation for allowed future CO2 emissions does not account for a global temperature target safer than 2°C, a higher chance of achieving the target than 66%, the warming effect of cutting aerosol pollution, or permafrost carbon release.
- It ignores the argument by some scientists that a safe level of CO2 is somewhere below 350 ppm (associated with ~1°C global warming above preindustrial).
- It does not lay out any example of the kind of emissions trajectory that would be required to meet a 140 GtC carbon budget. If we start steeply now, it would involve cutting global fossil fuel emissions by 6%/year.
- It contains little indication of the fears expressed by many scientists that the impacts of several degrees of global warming would be incompatible with the survival of human civilization as we know it. Policymakers reading the report may fail to fully grasp the scale of the climate change it projects. (CORRECTION: Upon reflection, I realize this is beyond the scope of Working Group I’s mandate.)
Although the IPCC does not make political recommendations, the implications of its conclusions could hardly be clearer. It is high time for our leaders to wake up and act, or for us to vote them out if they don’t. We urgently need to phase out global greenhouse gas emissions, most importantly fossil fuel CO2 emissions, as quickly as possible. We need to leave the vast majority of the Earth’s fossil fuel reserves in the ground, even just to have a good chance of limiting global warming to the unsafe target of <2°C.
The evidence ignored by the IPCC has implications too. It implies we are already entering a period of dangerous global warming and there is very little time to avoid large feedbacks that could send climate change spiraling out of control. We should now seriously consider geoengineering to remove CO2 from the atmosphere on a large scale and/or directly cool the planet, to keep the Earth from crossing the treacherous tipping points upon which we appear to be poised. However, given the myriad hazards of geoengineering techniques, they must be used only as a stopgap to treat the symptoms it is too late to prevent.
To give us any chance of avoiding catastrophe, our focus must be on addressing the main cause of our precarious situation: fossil fuel emissions.