A scientist says there is no longer such a thing as a purely natural weather event
Mike Hulme
With the immediate crisis in Mozambique gradually receding, now is the time to address the question I have been asked most frequently over these past two weeks by journalists, friends and acquaintances and even by my own seven-year-old daughter: “Were the floods caused by global warming?”
To answer this question, we need to distinguish between the meteorology of what happened – very heavy rains over several days across a very wide area – and the impacts of the rainfall upon river flows and the subsequent human dislocation and suffering.
Whether widespread heavy rain leads to extensive flooding depends on local factors such as land cover, dam management and flood control. And whether excessive flooding leads to widespread human suffering depends on where people live, what warning they are given and what options they have to remove themselves from risk.
Clearly, there are lessons to be learned from the events in Mozambique in both of these areas, but they are not lessons directly related to global warming. The more precise question we have to answer is: “Were the heavy rains over south-eastern Africa caused by global warming?”
This question does not lend itself to a simple “yes” or “no” answer. It is not possible to prove or disprove that a given severe weather event only occurred because of a rapidly warming global climate.
We can try to argue from first principles by saying that a warmer atmosphere will hold more moisture and, therefore, have the capacity to deliver heavier rainfall. Or we can argue that warmer ocean temperatures potentially provide tropical cyclones with increased energy to make them more vigorous and the rainfall more intense. But in neither case does this convince us that these particular heavy rains were caused by global warming.
Alternatively, we can try to argue from the statistical evidence. Have such intense rainfalls been recorded before in this part of the world? Well, yes they have – in the 1950s, for example. But have such heavy rainfall events become more frequent? For this part of Africa, the data has not yet been analysed with this question in mind and nor is the data particularly long or reliable enough to yield a convincing statistical answer.
This analysis, by the way, has been performed in some other regions – the United States, United Kingdom and Australia – and evidence has been found for a trend towards more intense daily rainfall totals in recent years.
For Mozambique, the definitive answer to our question continues to elude us. Indeed, it is quite possible that, in another 25 years’ time, even with the world 0,5C hotter than today, climate scientists will still be saying the same thing: “We cannot say that any specific extreme weather event is caused by global warming.”
If we approach our question from a different angle, however, then I believe there is something important to say about the relationship between global warming and extreme weather events – and this needs saying now, and needs repeating each time the need for action to mitigate climate change, or to adapt to its consequences, is questioned.
We know that the global climate is warming. We know that the rate of warming has accelerated in recent decades. Following the 1996 report by the United Nations’s intergovernmental panel on climate change, the balance of evidence reveals a detectable human influence on this warming.
A warming global climate will inevitably lead to changes in the behaviour of all weather systems – the systems that actually deliver weather to us, extreme or not. A global climate warmed by human pollution of the atmosphere must yield different magnitudes and frequencies of a whole spectrum of local weather events.
In this sense, therefore, the weather we now experience is the result of a semi- artificial climate; in a fundamental sense, it is different from the weather we would experience on a parallel planet which humans had not polluted. All weather events we experience from now on are to some indeterminable extent tainted by the human hand. There is no longer such a thing as a purely “natural” weather event.
This is the important message to take away from Mozambique, or indeed from any other extreme weather event, such as the heavy rainfalls in Venezuela before Christmas or the windstorms in France after Christmas.
We are running massive risks by altering the climate of our planet in ways we do not fully understand, let alone are able to predict with confidence. As with genetically-modified organisms, humanly- modified climates potentially expose us to risks that are largely unknown and unquantified. And the longer we continue to rely on a carbon-based energy economy, the greater these risks will be.
It will take more than the 20% reduction in greenhouse gas emissions announced on March 9 by the UK government to bring these risks under control.
Gambling with our climate may be acceptable to those of us living in affluent societies in the north, reckoning that we can adapt or buy our way out of climate trouble, but this gambling mentality is not one that would find many takers in Mozambique this week.
Dr Mike Hulme is reader in climatology at the University of East Anglia, Norwich, England