Climate Code Red

And 2017 “marks the first time some of the (scientific) papers concluded that an event could not have occurred — like, at all — in a world where global warming did not exist. The studies suggested that the record-breaking global temperatures in 2016, an extreme heat wave in Asia and a patch of unusually warm water in the Alaskan Gulf were only possible because of human-caused climate change”, Reuters reported.

Much of what happened in 2017 was predictable: news of climate extremes became, how can I put it … almost the norm. There was record-breaking heat on several continents, California’s biggest wildfire (extraordinarily in the middle of winter), an ex-tropical cyclone hitting Ireland (yes, Ireland) in October, and the unprecedented Hurricanes Harvey, Irma and Maria that swept through the Atlantic in August. The US government agency, the NOAA, reported that there were 16 catastrophic billion-dollar weather/climate events in the USA during 2017.

At both poles, the news continues to be not good. At the COP23 in Bonn, Pam Pearson, Founder and Director of the International Cryosphere Climate Initiative, warned that the cryoshere is becoming “an irreversible driver of climate change”.

As well, Pearson said that 2100 is a misleading and minimising measure of cryosphere response: “When setting goals, it is important to look to new irreversible impacts and the steady state circumstances. The end of the century is too soon to show that before but inevitable response especially for sea level rises.”

She said that most cryosphere thresholds are determined by peak temperature, and the length of time spent at that peak, warning that “later, decreasing temperatures after the peak are largely irrelevant, especially with higher temperatures and longer duration peaks”. Thus “overshoot scenarios”, which are now becoming the norm in policy-making circles (including all 1.5°C scenarios) hold much greater risks.

Pearson added that: “What keeps cryosphere scientists up at night are irreversible thresholds, particularly West Antarctica and Greenland. The consensus figure for the irreversible melting of Greenland is at 1.6°C.”

So what did we learn about the climate system in 2017? Here’s three that stand out, that should send shivers down the spines of policy makers.

1. 2017 was the second hottest year on record and the hottest non-El Nino year on record

Whilst not all sources have yet released data on annual warming for last year, the Copernicus Climate Change Service, the first major international weather agency to report global 2017 temperatures, said they averaged 1.2°C above pre-industrial times. 2017 was slightly cooler than the warmest year on record, 2016, and warmer than the previous second warmest year, 2015, Reuters reported.

Other organisations have unofficial figures which either agree with this assessment, or say that 2017 has tied with 2015. And last year was Australia’s third-warmest year on record.

It is no surprise that the last three years have been the hottest on the instrumental record. What is remarkable is that 2017 was as hot, or hotter than 2015, because 2015 and 2016 were both El Nino years, and the evidence shows that El Nino years are, on average, about 0.15°C warmer than La Nina years.

In fact, a remarkably hot 2017 crushed the old record for hottest non-El Niño year (2014) by an astounding 0.17°C.

The underlying temperature trend is being driven by continuing high levels of climate pollution: The UN says carbon dioxide levels grew at record pace in 2016. The atmospheric carbon dioxide averaged 403.3 parts per million (ppm) over the year, up from 400 ppm in 2015. The growth rate was 50 percent faster than the average over the past decade.

And global carbon emissions are headed up again after three years in which human-caused emissions appeared to be levelling off. A two percent increase is projected overall, with the highest rise coming in China, according to new research presented at the climate talks in Bonn.

In 2017 we also learned that there was no pause in global warming: the so-called ’slow down’ in climate change between 1998 and 2012 was caused by a lack of data from the Arctic.

2. It is likely to get hotter than we think

Two significant pieces of work released towards the end of 2017 suggest that warming is likely to be greater than the projections of the Intergovernmental Panel on Climate Change (IPCC), on which climate policy-making and carbon budgets are generally based.

This is because what is called Equilibrium Climate Sensitivity (ECS), an estimate of how much the planet will warm for a doubling in the level of greenhouse gases, is higher than the median of the IPCC’s modelling analysis.

In “Greater future global warming inferred from Earth’s recent energy budget” published in Nature in December 2017, Brown and Caldeira compared the performance of a wide range of climate models (raw model projections) with recent observations (especially on the balance of incoming and outgoing top-of-the-atmosphere radiation that ultimately determines the Earth’s temperature), in order to assess which models perform best.

The models that best capture current conditions (the “observationally-informed” models) produce 15% more warming by 2100 than the IPCC suggests, hence reducing the “carbon budget” by around 15% for the 2C target.

For example, they find the warming associated by the IPCC with RCP 4.5 emissions scenario would in fact “follow the trajectory previously associated with (higher emissions) RCP 6.0” scenario.

They also find that the observationally-informed ECS prediction has a mean value of 3.7°C (for a doubling of the atmospheric greenhouse gas level), compared to 3.1°C used in raw models, and in the carbon budget analyses widely used by the IPCC, the UN and at climate policy conferences.

In “Well below 2C: Mitigation strategies for avoiding dangerous to catastrophic climate changes”, published in September 2017, Xu and Ramanathan look at what are called the “fat tail” risks. These are the low-probability, high-impact (LPHI) consequences (“fat tails”) of future emission scenarios; that is, events with a 5% probability at the top end of the range of possible outcomes.

These “top end” risks are more likely to occur than we think, so “it is important to use high-end climate sensitivity because some studies have suggested that 3D climate models have underestimated three major positive climate feedbacks: positive ice albedo feedback from the retreat of Arctic sea ice, positive cloud albedo feedback from retreating storm track clouds in mid-latitudes, and positive albedo feedback by the mixed-phase (water and ice) clouds.”

When these are taken into account, the researchers find that the ECS is more than 40% higher than the IPCC mid-figure, at 4.5-4.7°C. And this is without taking into account carbon cycle feedbacks (such as melting permafrost and the declining efficiency of forests carbon sinks), and increase methane emissions from wetlands, which together could add another 1°C to warming be 2100.

This work compliments other recent work which also suggests a higher climate sensitivity: