Does 2°C make Earth uninhabitable?

In the spirit of making my blog name somewhat relevant to this module and the theme my blog is following at the moment, today I'm looking at arguments that a mere 2°C increase in temperature will render Earth uninhabitable for humans (and perhaps for other species).

There are many aspects of an environment that make it an appropriate habitat for a species. So I'm only going to look at a few different factors that I think are important for life to exist on Earth. For this blog post, I'll be looking at temperature as the basic limiting factor for habitability of regions for humans and other animals.

Below is a diagram demonstrating the range of temperatures and relative humidity that are comfortable for humans. The temperature range is from around 20°C to 30°C, and is an indication of indoor air temperature, but depends on relative humidity as well. This makes sense, as 30°C with relative humidity of 60% would be highly uncomfortable, but 30°C with humidity of 30% would be more comfortable. Humans maintain a core body temperature of around 37°C, even within highly variable climates; and human skin is strongly regulated at 35 °C or below, because the skin must be cooler than body core in order for metabolic heat to be conducted to the skin. Sustained skin temperatures higher than 35°C imply hyperthermia, which can be dangerous for health.




Outdoor air temperatures endured by humans around the world today are obviously a much larger range than the graph above. However, these temperatures are neither sustained naturally nor are humans exposed to these temperatures for extended periods of time. Humans have also found adaptations for living in extreme temperatures, such as air conditioning and central heating. Even with these adaptations, mounting evidence indicates that temperatures will only get higher, and the Met Office even claims that 2015 is likely to be the hottest year on record, and on average 1°C than pre-industrial levels.

So how much is rising temperature a threat to human survival? An article in CNN recently ran this sensationalist headline: "Persian Gulf heat: It may become too hot for humans to survive, study warns", and the study cited, focuses on southwest Asia, a region already experiencing extreme temperatures. The study claims that many cities need to support global mitigation strategies to enable the IPCC climate change projection pathway of RCP4.5, rather than RCP8.5 which is the business as usual approach. If this happens, under simulations, the maximum temperature threshold is never reached, and although temperature will be higher, these regions would not be uninhabitable. However, they claim that near the cities of Jeddah and Mecca, important religious grounds, are a mere 2°C away from reaching the temperature threshold, and therefore there are certainly vulnerable areas that require immediate action to prevent the warming that would otherwise occur.

The authors of the study, Jeremy Pal and Elfatih Eltahir, base a lot of their assumptions about the temperature threshold on another study, by Steven Sherwood and Matthew Huber. This study looks at the limit of adaptability to climate change due to heat stress, as measured by wet-bulb temperature, which is a measure of both temperature and humidity. Their study uses six-hourly 2-metre temperature, humidity and pressure data from the ERA-Interim dataset to determine wet-bulb temperature, which so far never exceeds 31°C. Based on the assumption that human skin temperature of 35°C for a sustained period of time would cause hyperthermia, they suggest that a wet-bulb temperature of 35°C is the upper limit for adaptation to climate change, as humans and other mammals would not be able to dissipate metabolic heat. Therefore, if a region had a temperature of 35°C for an extended period of time, it would be "unlivable". Sherwood and Huber also stress that adaptations such as air conditioning would not be "satisfying, affordable, and effective for most of humanity" as power shortages would be life threatening, and humans would be limited to staying in the safety of air conditioned areas.

While I find Sherwood and Huber's study elegant, it concludes that an average global warming of 7°C would be needed in order for there to be areas with 35°C wet-bulb temperatures for an extended period of time. A warming of 11-12°C would encompass most of the areas humans live in now. Therefore this average warming needs to be much higher than 2°C in order to have dire effects. But this study shows how temperature in isolation can be a threat and that this potential result of climate change deserves some attention on its own. They emphasise this with the statement: "If warmings of 10 °C were really to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level".

These two studies on temperature as a limiting factor for the habitability of areas on Earth serve to stress the importance and urgency of finding a solution to limit mean warming to as low as we possibly can, and of implementing mitigation measures as soon as possible. This is all too relevant in the lead up to COP21 in a mere 4 days and I'm sure the world will be waiting with bated breath for the results.

Next week I look forward to exploring other impacts of a 2°C rise in temperatures and whether they might render Earth uninhabitable!

P.S. Another interesting idea is that climate change can make Earth uninhabitable because it can be a cause of conflict. Earlier this week, Sky News released a video interview with Prince Charles, where he mentioned the 2 degrees threshold, and how the drought in Syria is partly to blame for the conflicts there. I won't be exploring this further as I don't believe it's possible to quantify the impact of climate change or warming on conflict, but it's interesting to consider that climate change can affect complex human interactions.

Using energy to limit 2 degrees

In my last post, I spoke about the 2°C rise limit that is more or less agreed will limit "dangerous" consequences of climate change. In reality there is no hard limit but nonetheless it's an important figure for political, social and scientific reasons. The more difficult part to quantify is the question of how we will endeavour to prevent 2°C rise. There are many areas that could fall into the answer to this but this blog post will focus on energy, and therefore fossil fuels.

Since I've mentioned the IPCC Fifth Assessment Report in almost every other blog post, let's begin by looking at their chapter on Energy Systems in the report on Mitigation of Climate Change. The reason energy is included under mitigation of climate change is split into 5 sections: Fossil fuel extraction, conversion, and fuel switching; Energy efficiency in transmission and distribution; Renewable energy technologies; Nuclear energy; and Carbon dioxide capture and storage (CCS). They state that "climate change can only be mitigated and global temperature be stabilized when the total amount of CO2 emitted is limited and emissions eventually approach zero", and that the energy supply sector is the largest contributor to global greenhouse gas emissions, hence energy places a large role in preventing the 2 degrees scenario from being reached.

The ways energy can aid the prevention of the 2 degrees scenario include decarbonisation, increasing energy efficiency and geoengineering. The Carbon Brief's article is an excellent summary and brief introduction to these ideas.

1) Decarbonisation of energy

This method means moving towards a system that uses energy that does not emit any carbon dioxide to the atmosphere and therefore will significantly reduce the human impact on climate. General discussion of energy in the context of climate change focuses on this method, and particularly on transitioning from a fossil fuel dependent world to a fossil fuel independent world through the use of renewable energy. Although this seems straightforward enough, the presence of fossil fuels still remains massive on a global scale, and many economies and societies have not ventured into decarbonisation just yet. There is definitely hope though, as fossil fuels and renewable energy are currently hot topics, and with COP21 on the horizon,steps have (hopefully) been taken to include decarbonisation on the agenda for many countries and their policies.

The role of National Geographic and many other climate focused organisations and websites such as the Carbon Brief help to put a spotlight on the developments in decarbonisation of energy and wider climate issues. For example a recent article from National Geographic examines the cities closest to running entirely on renewable energy sources. It's an important reminder that, amidst the widely broadcast country level energy and policy discussions, change can and is already happening at the city level and at even smaller scales.

The Carbon Brief is a great source of information on studies and other scientific discoveries that are accessible to the general public and the focus on the role of carbon in global climate change, much like this blog! In an article, they synthesise the study of Christophe McGlade & Paul Ekins on the role of coal in the energy industry for the future. McGlade and Ekins suggest that, in order to stick to under 1100 gigatonnes of carbon dioxide emitted between 2011 and 2050 (I discussed this idea of a carbon budget in the last blog post) and thus within 50% chance of staying under 2°C rise, 1/3 of oil reserves, 1/2 of gas reserves and over 80% of current coal reserves should remain unused. If we ignore this and use all of the world's known oil, gas and coal reserves, the carbon budget would be three times the recommended amount. The graphic below shows a breakdown by 10 specific regions.

Interestingly, the paper discusses the use of gas as a way of transitioning to a carbon-free world, as approximately 94% of Europe's gas reserves are "burnable" under a 2 degree scenario. However on a global scale, meeting the 2 degree commitment in a cost-effective way means leaving 100% of unconventional oil and 82% of unconventional gas resources unburned, according to the model. These numbers emphasise the need for a global commitment to transition to low or zero carbon energy, and quickly.

The good news is, this is the expected future. The International Energy Agency published their medium-term report in October 2015 that forecasts that 26% of the world's energy supply will come from renewable energy sources by 2020 (a mere 5 years away!). Sweden leads the way towards becoming the world's first fossil-free nation, through increasing the budget for renewable energy. China, though currently the world's biggest producer of carbon emissions, is responsible for 40% of global renewable capacity growth, and with the help of global commitments, the desire to have energy security, as well as economic incentives, many of the world's worst carbon emitters could be turning things around.

2) Increasing energy efficiency

Along with decarbonisation, another method that could help mitigate climate change of 2°C is increasing energy efficiency. With growing technology, this may seem straightforward; however the global population and therefore demand for energy is still increasingly rapidly. By 2040, the World Energy Outlook estimates that total population will hit 9 billion, and global wealth is expected to double. Without any policies, this growth would lead to a 50% increase in energy usage, but if we are to keep to the 2 degree scenario, energy demand can only increase by 17%. In order to limit energy use, the world would need to become twice as productive. Regulations play an important role here, with policies needed to drive energy efficiency, such as ending fossil fuel consumption subsidies and consumer product efficiency standards to be enforced.

A good example of this is in the recent Volkswagen scandal (although with nitrous oxide not carbon dioxide). Had Volkswagen adhered to the Environmental Protection Agency's standards for carbon emissions from their cars, a potential 237,161 and 948,691 tonnes of NOx emissions could have been avoided, according to The Guardian.

Diana Ürge-Vorsatz & Bert Metz summarise the role of energy efficiency in a paper in the aptly named journal Energy Efficiency. They state that energy efficiency has played a key role in reducing society's CO2 emissions, particularly in the last decade, and will likely continue for the forthcoming decades. The paper draws on the IPCC's Fourth Assessment Report and stipulates that improved efficiency is likely to be a key focus in the short term, before shifting to low-carbon energy sources as costs for energy improvement grow while decarbonisation costs decrease in the longer term. The sectors with the biggest potential for energy efficient are building, transport and industry.

Therefore because in the planning and preparation for a zero carbon future, steps must be made to transition easily and with as little economical impact as possible, energy efficiency is an important player.

3) Geoengineering

Carbon capture and storage (CCS) is a method of removing carbon emissions before they are released, which could enable negative carbon emissions (released, not necessarily cumulative, it must be noted). This is directly related to energy as it utilises fossil fuel plants and has the greatest potential for use in the energy sector. I intend to explore this topic in more depth in the future but this method of mitigation has the potential to alleviate some of the pressures of rapid decarbonisation that is needed to fit within the 2 degree scenario.

4) Other mitigation solutions

There are potentially a whole multitude of ways to mitigate climate change to stick to 2 degrees that have yet to gain traction or yet to be discovered. A simple one would be to enforce reductions of energy usage, irrespective of the source, which is hopefully on the agenda for COP21, and the many technologies evolving and advancing as we speak (well as I type) can definitely help. One of my many hopes for the future is the transition to a world that only uses electric cars, led by the brilliant Elon Musk and Tesla (have a read of this article, if you have some time to spare).

I hope this gave some insight into how energy plays such a key role in our journey to keep temperatures below the all-important 2°C. There is still a lot to be done before we can wipe our brows and breathe a sigh of relief that we are no longer on the path to an uninhabitable planet, but if changes are put into motion soon (like COP21 soon), there is undeniably some hope for humanity.