SOURCE: Nature
DATE: December 5, 2018
SNIP: In October, the Intergovernmental Panel on Climate Change (IPCC) released a report setting out why we must stop global warming at 1.5 °C above pre-industrial levels, and how to do so. It is grim reading. If the planet warms by 2 °C — the widely touted temperature limit in the 2015 Paris climate agreement — twice as many people will face water scarcity than if warming is limited to 1.5 °C. That extra warming will also expose more than 1.5 billion people to deadly heat extremes, and hundreds of millions of individuals to vector-borne diseases such as malaria, among other harms.
But the latest IPCC special report underplays another alarming fact: global warming is accelerating. Three trends — rising emissions, declining air pollution and natural climate cycles — will combine over the next 20 years to make climate change faster and more furious than anticipated. In our view, there’s a good chance that we could breach the 1.5 °C level by 2030, not by 2040 as projected in the special report (see ‘Accelerated warming’). The climate-modelling community has not grappled enough with the rapid changes that policymakers care most about, preferring to focus on longer-term trends and equilibria.
Three lines of evidence suggest that global warming will be faster than projected in the recent IPCC special report.
First, greenhouse-gas emissions are still rising. In 2017, industrial carbon dioxide emissions are estimated to have reached about 37 gigatonnes. This puts them on track with the highest emissions trajectory the IPCC has modelled so far. This dark news means that the next 25 years are poised to warm at a rate of 0.25–0.32 °C per decade. That is faster than the 0.2 °C per decade that we have experienced since the 2000s, and which the IPCC used in its special report.
Second, governments are cleaning up air pollution faster than the IPCC and most climate modellers have assumed. Mainstream climate models had expected them to rise. Lower pollution is better for crops and public health. But aerosols, including sulfates, nitrates and organic compounds, reflect sunlight. This shield of aerosols has kept the planet cooler, possibly by as much as 0.7 °C globally.
Third, there are signs that the planet might be entering a natural warm phase that could last for a couple of decades.
These three forces reinforce each other. We estimate that rising greenhouse-gas emissions, along with declines in air pollution, bring forward the estimated date of 1.5 °C of warming to around 2030, with the 2 °C boundary reached by 2045. These could happen sooner with quicker shedding of air pollutants. Adding in natural decadal fluctuations raises the odds of blasting through 1.5 °C by 2025 to at least 10%. By comparison, the IPCC assigned probabilities of 17% and 83% for crossing the 1.5 °C mark by 2030 and 2052, respectively.
Various climate engineering options should be on the table as an emergency response. If global conditions really deteriorate, we might be forced to extract large volumes of excess CO2 directly from the atmosphere. An even faster emergency response could be to inject aerosols into the atmosphere to lower the amount of solar radiation heating the planet, as air pollution does. This option is hugely controversial, and might have unintended consequences, such as altering rainfall patterns that lead to drying of the tropics. So research and planning are crucial, in case this option is needed.