The last drop of rain has still to fall from ‘Harvey’, but the claims and counterclaims are already flying in both directions of the climate debate. To what extent can this storm be linked to climate change? The buzz word here is ‘attribution’.
Just as one sneeze does not prove an illness, so individual weather events, however extreme, cannot prove climate change (man-made or otherwise). The atmosphere is every bit as complicated as human physiology, except we probably know even less about the former. That makes diagnosis (or attribution) rather difficult.
A warmer atmosphere can hold more moisture
In terms of wind strength, Harvey was merely the 14th most powerful hurricane on record to make landfall in the United Sates. It was energised by the warm waters of the Gulf of Mexico, which have been warmer than normal this summer. However there are many other atmospheric factors that go into storm development. Indeed, as evidence of this, despite the overall warming trend over decades, there has actually been a distinct dearth of storms making US landfall in recent years. There’s certainly no obvious trend upwards here or, indeed, elsewhere in the world.
Globally, the atmosphere has warmed by as much as 1C since pre-industrial times. A warmer atmosphere can hold more moisture – in fact billions of tonnes more water vapour which, given the right weather situation, can be deposited as rain. Logically, then, rainfall events have the propensity to be more intense than previously. Yet it was the slow-moving nature of Harvey, much more than its intensity, that caused the phenomenal rainfall totals of well over a metre – a year’s worth – over several days.
Ironically, it was actually the existence of an unusually cold wedge of air over the Plains and the Midwest that blocked Harvey’s path, leading it to stall and deposit so much rain in the same area for so long.
Flooding is often at its most deadly where the land meets the sea, particularly when colossal river outflows meet an incoming storm surge. The water literally has nowhere to go but up, a factor made worse if background sea levels are rising too. And as ice caps and glaciers melt, sea levels are indeed rising – already by tens of centimetres. For a given storm and a given tide, coastal flooding is becoming more frequent and widespread.
… the patient is already running a temperature
However, urbanisation, clearance of vegetation and poor water-management strategies have exacerbated the vulnerability of many coastal communities to flooding. Furthermore, around the Gulf coast, mining and drilling operations have led to land subsidence in some areas. This all muddies the waters, so to speak.
So what to conclude? The inconvenient truth is that this catastrophe neither was, or was not, evidence of our warmer world because such simplistic statements are impossible to make. The impacts of Harvey were probably overwhelmingly the result of a natural quirk in the weather pattern over North America. But on top of this, human factors probably contributed further to the impacts of this ‘perfect storm’.
Experts will be frantically researching the data to ascertain that human-induced X-Factor – that made Harvey just so impactful. In the academic game of attribution, I anticipate a flurry of numerical estimates – all of which will be far too precise, given how little we understand about the climate system and its nuances. But such research, without hidden political or egotistical agendas, is so important in building up our knowledge-base of the climate, so that we can predict into the future and mitigate against likely impacts.
Returning to the medical analogy – the patient is already running a temperature and if its sneezes grow in number and intensity, then we should be increasingly concerned. Frustratingly, we still know surprisingly little about this patient called the climate. But our knowledge is growing and we should at least be intelligent enough to do all we can to look after it. It’s the only patient we’ve got.