By Toryn Whitehead
1.5°C may not sound like a lot, but more frequent and extreme weather events such as flooding, droughts and wildfires are pushing people to the brink around the globe. So, why are scientists slow to attribute a single extreme weather events to climate change?
Do you remember that really hot summer in 2019? In Cambridge, temperatures reached 38.7°C – the UK’s highest daily maximum temperature on record. But was this record breaking weather related to climate change? Due to the chaotic nature of weather it is difficult to say for any specific event that it would not have occurred in absence of climate change. This is why you rarely hear scientists say this weather event is because of climate change. To be able to make that leap, you need to zoom out and look at the bigger picture.
The bigger picture
The world is becoming more hostile. Temperatures are rising, annual rainfall is increasing and the seasons are shifting. Although these changes vary in different geographic locations, the trend is undeniable.
1.5 °C may sounds small right? For example, if you heard on the radio that the temperature today was supposed to be 20 °C, but your phone said it was 21.5 °C, this difference probably wouldn’t lead to any behavioural changes. We would all still go to the park to meet our friends and enjoy the good weather.
Yet, in terms of global climate change and the frequency and severity of weather events, the consequences of 1.5 °C warming is huge. We currently at about 1.16 °C warming today compared to pre-industrial (1850-1900) levels used by the IPCC as a baseline for warming. Already the polar ice caps are shrinking at an alarming rate leading to sea levels rising which threaten the very existence of coastal cities such as Jakarta, Indonesia’s capital (for now). Increased ocean surface temperature is resulting in longer and more powerful hurricanes in Central America and Southern US. Drier conditions are causing more frequent and extreme wildfires such as those seen in Australia last year. Yet, despite the destruction, economic fallout and lives lost, it is still debated whether climate change is the primary cause.
There is some debate regarding climate projections for rainfall over the coming decades, but on average, dry areas will become drier and wet areas wetter. Importantly, wherever you are, rainfall extremes will become more intense. Models predict a 16 – 24% increase in heavy rainfall by 2100. This is something we are already experiencing in the UK, and as the risk of flooding rises, so does the impact on society. One study has projected that under a high emissions scenario, the annual cost of UK flooding could increase 15-fold by 2080. To put that in context, the flooding that struck the UK a year ago between February 9th and 29th cost around $500 million. The data paints a bleak picture, but what other factors are at play? And to what extent is climate change responsible for this shift?
Clearly climate change is playing a significant role, but weather systems are complex and disentangling the multitude of factors contributing to an extreme weather event is difficult.
For example, returning to Jakarta, human factors have certainly contributed to the city’s impending doom. Overpopulation in the city has led to an increased demand for fresh water. However, the city has been unable to meet the high demands, resulting in locals pumping water from the ground – otherwise known as groundwater. This leads to land subsistence, so as water is pumped out from the ground the land above sinks, as so do the surrounding buildings. This has exacerbated Jakarta’s vulnerability to climate-related disasters such as flooding, highlighting the role of factors beyond human-induced climate change. Therefore, how do we know how big a role climate change is actually playing? Could the cause of weather events be more localised as a result of local environmental degradation?
Probabilistic event attribution
With uncertainty surrounding the relationship between human-induced climate change and specific climate disasters, determining the extent to which climate change is the cause of these more frequent and extreme weather events is important. Attribution studies, a relatively new area of research, attempt to resolve this issue by answering the question “Would this event have occurred in the absence of climate change?”.
To do that, you need to collect a ton of data and run computer models to simulate the earth’s climate based on two different scenarios, with the first model simulating today’s climate, accounting for human-induced climate change, and the second model simulating a climate close to what existed pre-industrial revolution before humans burnt fossils fuels and caused the concentration of greenhouse gas emissions to rocket. Comparing these simulations allows us to see the effect of climate change on a specific event.
This is just what Robert Vautard and colleges did for the heatwaves in Western Europe in 2019. They found that without climate change, this event would have been extremely improbable. Furthermore, in the event that these heatwaves occurred without human-induced climate change, temperatures would have been 1.5 – 3 °C lower. Although this may not seem like a lot, such temperature rises result in a steep rise in morbidity and mortality.
For example, in the UK last summer, heatwaves caused a record 2,556 excess-deaths in the UK. So despite many of us enjoying the longer, hotter summers, it is clearly starting to exacerbate people’s underlying health issues – endangering old and obese people the most.
92% of attribution studies looking at extreme heat around the world have concluded that human-induced climate change made the event more likely or more severe. Confirming not just that climate change is real, but that it is the primary cause of the more extreme and frequent weather threatening societies around the world. Human factors such as the problem of land subsistence in Jakarta merely intersect with climate change to worsen climate-related disasters, which in themselves are more extreme.
So next time you see news headlines about forest fires in the Yorkshire moorlands or flooding in South Manchester, posing the question “Did climate change cause this?” The answer is yes. Without a doubt.
Graphic courtesy of Alice Eaves