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...

The world's biggest hoax? Part 2

This post follows on from last week’s post and aims to answer the question: does evidence support the idea that human activity has played a significant role in this change?

Most people have probably heard the story of CO₂ and the greenhouse effect, but here’s a diagram to recap.



Figure 4: How greenhouse gases like CO₂ cause the Earth to warm.

You may well have seen in the news recently that CO₂ reached a permanent concentration of 400ppm in theatmosphere, which probably means nothing to you until I tell you that’s the highest it’s been for over 800,000 years. The IPCC states that CO₂ is the largest single contributor to increases in temperature between 1750 and 2011.

As is commonly pointed out by sceptics though, CO₂ has many natural sources- so who’s to say us burning fossils fuels is to blame? Well, apart from the fact that we know we’ve burnt enough fossil fuels to contribute that much CO₂ to the atmosphere, CO₂ from fossil fuels has a chemical signal that means it can be identified.

Box 2: The Suess Effect (for those who like chemistry) Carbon has three naturally occurring isotopes: C12, C13 and C14. During photosynthesis, the different carbon isotopes are not absorbed with equal preference, leading to plants and algae being depleted in C13.  As fossils fuels were once live organisms, they too carry this isotopic signal. C14 is also depleted in fossil fuels as it is radioactive. As its half life is close to 5000 years, over the millions of years fossil fuels take to form most has decayed. By monitoring changes in the ratios of different carbon isotopes in the atmosphere we can calculate the contribution of CO₂ from fossil fuels. This was first noted by Hans Suess, and so is named the Suess effect.

It’s not just fossil fuel combustion either- greenhouse gas emissions (22% of the total according to the IPCC) also come from other human activities, such as methane from farming and CO₂ from deforestation.

The above evidence has contributed to the IPCC being able to state that ‘It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.’

In other words, the very best interpretation of all the very best evidence by the very best collaboration of climate scientists suggests that we are causing the most extreme climate change seen on Earth for thousands, if not millions of years. I can’t argue with that.

The world's biggest hoax? Part 1

In future I’m going to have a look at specific, recent stories about climate change in the media, but I thought I’d write the first proper post here on arguably the biggest question surrounding anthropogenic (human-caused) climate change: is it really happening? This is a big topic so I’m going to split it into two posts.

The arguments against anthropogenic climate change come in several forms. Some deny that climate change is happening:

Snowing in Texas and Louisiana, record setting freezing temperatures throughout the country and beyond. Global warming is an expensive hoax!

— Donald J. Trump (@realDonaldTrump) January 29, 2014

Others, such as more than half the US Senate, recognise that the climate is changing but refute humanity’s role. Many point to past changes in both climate and CO₂ as evidence that current changes are natural or within ‘normal variation’, and deny that there is a scientific consensus regarding our role.

Meanwhile many media outlets regularly publish articles predicting the extreme and imminent effects of climate change, such as destroying infrastructure, worsening extreme weather events and even endangering US military operations. I hope to have a look at some of these in a later blog.

Much has been written for and against these arguments, and I can’t cover the nuances of every point in just one post. I would really recommend the Skeptical Science blog for a more detailed discussion of specific questions (thanks to Anson for the recommendation).

For now though, I will try to answer whether climate change is all a hoax by simplifying it to two key questions:

1) Does evidence support the idea that climate is changing outside the bounds of ‘normal variation’? (This post)
2) Does evidence support the idea that human activity has played a significant role in this change? (Next post)

Firstly, I’d like to share this awesome graphic from NASA. 

Figure 1: Surface temperature changes from 1880 to 2015, using a rolling five year average. Blue colours represent temperatures cooler than the 1951-80 baseline, while red colours represent temperatures above that baseline.

It seems pretty clear that temperatures have risen over the last century-and-a-bit.

Bloomberg plotted NOAA data to show a similar trend. They also highlighted the fact that 15 out of the 16 hottest years on record have been this century, and that 2015 was not only the hottest year on record, but also beat the previous record by a record margin (N.B. NASA’s data suggests the increase from 1997-8 was larger, but corroborates the fact that 2015 was the hottest year on record).

I’d say these data sources are fairly reliable. However, arguably the most reliable sources of data relating to climate change are the IPCC reports. 

Box 1: The IPCC The Intergovernmental Panel on Climate Change is Nobel Prize-winning body, established by the UN to improve understanding of the scientific basis of climate change. It doesn’t carry out its own research, but instead reviews published papers (both peer- reviewed and non peer-reviewed). Based on this evidence, it then reaches conclusions, and, most importantly and interestingly I think, attaches a degree of certainty to these. Although the IPCC has received much criticism in the past, for example over claims about the disappearance of Himalayan glaciers it is generally considered to give a fair reflection of the scientific consensus on climate change. The most recent report, its 5th, was published in 2013.

So what does the IPCC have to say about whether the climate is warming? In the most recent report it states that ‘the globally averaged combined land and ocean surface temperature data... show a warming of 0.85 [0.65 to 1.06]°C²⁰ over the period 1880 to 2012, for which multiple independently produced datasets exist’ (the figures in square brackets are the error margins). 

To me, this all seems fairly conclusive, but let’s look at one of the most popular arguments of climate change sceptics: ‘the great pause’. As the picture below demonstrates, it is a popular claim that temperatures have stopped increasing since the late 1990s.

Figure 2: The Sunday Mail’s headline and graph from October 2012.

As we saw above, warming hasn’t stopped in the last decades. The climate system is complex, and human impacts are modulated by and interact with natural cycles, such as El Niño events (which contributed to the highs in 1997/8 and 2015). We therefore don’t see a totally regular, noise-free pattern, but the short term variation does not negate the long term trend. In fact, a recent Nature paper found that hiatuses like this one are statistically inevitable when such small samples are considered.

However, even if we accept warming is occurring, this is only in the context of 130 years. We know the climate has fluctuated much more wildly in the past, with both ice ages and the sweltering conditions that accompanied the dinosaurs firmly in the public imagination. So why are modern changes any different? The IPCC states with high confidence that ‘1983 to 2012 was very likely the warmest 30-year period of the last 800 years in the Northern Hemisphere’ but this still barely registers on geological time scales. However, if we do look at a longer time scale, such as the Holocene (the last 11,000 years), it is easier to see the significance of recent changes. It is not the magnitude but the rate of warming that is alarming.

Figure 3: Adapted from Marcott et al. 2013 by www.realclimate.org. 

So far, so good (or bad for the inhabitants of planet Earth). But what about our role? Check out my next post to find out!