Climate pessimism

This week there has been a lot of pessimism regarding climate change in the news, largely due to the result of a certain election. For the sake of my sanity we are not going to dwell on said result, but instead look at a different gloomy story in the news this week. The Independent summed up the media stories well with this headline: ‘Climate change may be escalating so fast it could be 'game over', scientists warn. New research suggests that the Earth’s climate could be more sensitive to greenhouse gases than previously thought, raising the spectre of an ‘apocalyptic side of bad’ temperate rise of more than 7C within a lifetime’.

So are scientists actually warning this? The news articles are based on a paper published in Science Advances this week. The scientists in question were aiming to test current projections of how the Earth’s climate system will respond to increasing CO₂ using data about past climates.

They reconstructed a global surface air temperature record for the last 784,000 years, using sea surface temperature proxies from across the world. They then correlated this to records of atmospheric CO₂ (from air bubbles trapped in ice sheets), and used this to assess the sensitivity of the Earth’s climate system to changing CO₂ levels.

Box 1- SST proxies On much of the ocean floor sediment builds up gradually and constantly over time. This sediment often contains indicators of conditions at the time it was deposited- a climate proxy. A commonly used example (including in this paper) is the calcite shells of microorganisms called foraminifera. Depending on the species, the chemical and physical make up of the shell can give us a lot of information about the climate and water the shell formed in. The ratio of different oxygen isotopes (commonly written as δ18O) can give information about temperature and ice volumes at the time, as can other relationships such as the ratio of calcium to magnesium or the direction the shell coils in. By taking cores of sediment containing these proxies, scientists can reconstruct an accurate picture of how climate changed over the period the sediment was being deposited. Henderson (2002) provides a more detailed discussion of a number of proxies if you’re interested in learning more.

They found that there results were generally in good agreement with the projections they looked at. These were from CMIP5 (the coupled model inter-comparison project 5) which provided the projections used in the 5^th IPCC report  (see previous blog post) . It’s worth noting that all projections come with significant uncertainty- we don’t really know how things like cloud formation will feedback into climate responses. This is why when you see projections on graphs there is not a thin line, but a thicker band. The results from this study are well within the CMIP5 uncertainty range, but the maximum emissions scenario shows temperature increases by 2100 around 16% higher than the mean of the projection.

Figure 1: CMIP5 projections based on different emissions scenarios (here called RCPs). RCP8.5 is the projection referred to in this paper.

This is where the headlines come from. The study found that as the climate warms it becomes more sensitive to CO₂ i.e. that there is more warming per unit of CO₂. This might put the Earth at risk of ‘runaway’ climate change- where positive feedbacks amplify warming and the system spirals out of control. 7˚C would indeed be catastrophic, but these projections are based on high emissions scenarios where we do nothing to reduce how much CO₂ we produce. If we stick to our 2˚C ambitions we stand a good chance of avoiding runaway change.

One criticism of this paper is that over the period studied CO₂ levels have never been as high as they are today, never mind how high they would be in the worst case scenarios referred to in the headlines. They therefore cannot test climate responses to the type of climate forcing expected later this century. However, this paper finds good agreement with previous studies looking at longer time periods in which CO₂ was higher, for example Köhler et al (2015), who looked the last 5 million years.

So the two main takeaways from this post (apart from never trust a headline) are:

1.       There is more and more evidence that in the past climate responded to increasing CO₂ in a similar way to the way CMIP5 models suggest it will in the future. This means we have more confidence in the projections in the IPCC report for example. So if we stick to the 2˚C pathway we're in for a decent shot of avoiding the apocalypse.

2.       The warmer the world gets, the greater the effect additional CO₂ emissions will have on climate, and the more uncertain we are about its impacts. The more we emit, the higher the risk that the climate system spirals out of control. Even more of a reason to cut emissions fast!

Let’s hope the world’s leaders are ready to take bold actions to save the planet. Oh wait...