Can we say climate change ’causes’ extreme weather events?

October 14, 2009

     By Duncan Green     

Every time a flood, cyclone or drought makes it into the media, my colleague John Magrath is asked whether climate change is to blame. In a valiant attempt to avoid the researcher’s reflex but annoying ‘it’s more complicated ketsana 2than that’ response, he has produced this briefing.

‘There’s a natural tendency to blame major disasters solely or largely on climate change but I think it’s problematic for several reasons. One being that we’ve tried to discard the concept of “natural disasters” (akin to “acts of God”) to show that, whatever triggers an event; it’s human factors that largely dictate the casualties/damage. We’ve particularly tried to move the public away from blaming “the weather” for causing a disaster (Ethiopia 1984 etc). I worry that concentrating on climate change risks bringing the weather back into the discussion. Also, citing climate change every time could be like crying wolf, making it harder to kick up a fuss when unambiguous phenomena do happen. So it’s important to discriminate (though not easy). Here goes…

The most unambiguous climate change/global warming impacts so far are not sudden extreme events but long-term chronic, insidious ones, and tend to be in a) high latitudes and b) high altitudes, particularly:

* Melting glaciers (Nepal, Tajikistan, Bolivia, etc)
* Melting Arctic ice (and Greenland) – a global “average” temperature rise of 0.8 degrees translates into a lot more towards the north pole (not the south pole)
* Hotter Siberia.
* Possibly, the long-term drought in Australia

The more insidious events include:

*  More intense regular, seasonal rains (not cyclones, which are the next notch up)
* More concentrated rains – fewer refreshing showers pre-and post- main rains
* Hence longer dry/drier seasons
* Heatwaves (a greater probability thereof, like Europe 2003) and generally hotter temperatures, hence
* Shorter growing seasons for crops

Don’t forget that a poor and vulnerable farmer only needs something small to have their life devastated – a drought that lasts 3 weeks longer than “normal”, a storm that’s 5% bigger…and so on.

Climate change is a trigger and a magnifier/multiplier of natural phenomena, but climate change is only one cause – usually a first trigger – in a chain of events that “cause” disasters, especially floods and even to some extent, droughts, thus:

Floods: More intense, unexpected rains, especially localized, can trigger massive water flows. But as we know from UK experience, the occurrence, extent and duration of floods is more to do with water management, i.e. how fast the water runs off downstream etc. In urban areas, a lot of flooding is likely to be because drainage systems haven’t caught up with urban (usually slum) expansion, and what drainage there is, is blocked. There will be more floods associated with climate change, but it’s simplistic/dangerous to forget/downplay these other factors.

Droughts: We’re seeing growing aridity in e.g. Australia and in East Africa where the spring rains have declined over at least the last decade. These may well be linked to global warming changing weather patterns. But, how and when a “drought” is declared may be as much about water management (or lack of it) as rainfall. What constitutes a drought disaster is not just a meteorological drought (low rain over time), but has to do with who gets what available water, who stores/hoards available water (countries upstream? rich farmers upstream? cities and not rural users?), and the state of water storage facilities. This links to our concern for equity and rights.

Tropical cyclones, hurricanes etc: We seem to be getting more intense ketsana 1regular rains – but not necessarily stronger cyclones. Some scientists offer evidence that hurricanes have increased in intensity (but, n.b. not frequency) over the last 50 years. Others worry this is about changes to how you measure what, but just at the moment in the last 2-3 years the number of hurricanes has actually dropped to a 30-year low. The number of tropical cyclones – the next level down – has stayed the same.  Global warming may actually make for fewer hurricanes by creating stronger winds that – paradoxically – instead of feeding the hurricane will chop the tops off (like strimming) and stop them forming properly. Again – hurricanes don’t automatically equate with “disaster”. Even tropical storm Ondoy – that became typhoon Ketsana – wasn’t a very big storm; it just happened to hit the capital of the Philippines (cf flooding, above). But you only need relatively small increases in storms to have a devastating effect on people in flimsy boats and houses. For me it’s more convincing that in many places (Bangladesh and the Bay of Bengal, Vietnam, Haiti, Malawi and Indonesia) people report more  “regular” storms, high winds, rough seas – “under the radar” events that curtail fishing days and knock roofs off houses etc.

Sea level rise (SLR) and saltwater ingress: This is/will be deadly for many small island states and highly populated deltas. But with deltas, is the problem SLR, or sinking land? Deltas are highly populated so industry and agriculture suck up ground water, the land compacts and shrinks/sinks. Big dams upstream mean there’s a) less silt coming down to rebuild the deltas and b) less freshwater coming down to keep the salt water out.

Malaria: Temperature rise does encourage mosquitoes to live faster – move more, bite more, breed faster etc but some temperature rise could actually damage the parasites, and a lot of malaria spread is to do with human activity, e.g. deforestation leads to standing water (in wheel ruts) and sunny clearings. Also, some healthcare professionals worry that blaming climate change for malaria spread lets negligent governments off the hook for stopping spraying or abandoning preventative (or curative) health care for victims i.e. malaria spreads because governments let it.  

Global and local causes – temperature vs rain: Global warming is unambiguous, though the global climate system is like lumpy porridge: warming isn’t uniform everywhere. Warming has knock-on effects on plants, animals and humans. But temperature is only one cause of rainfall (think topography, altitude, vegetation). So that’s why it’s even harder to say “global warming is doing/will do this” when it comes to rainfall patterns.

Then there’s global warming and local warming. Broad changes to the seasons may be more to do with greenhouse gases but intensity of rainfall may be as much to do with local changes, especially deforestation.  The IPCC says that local vegetation cover can make a difference of 0.8 degrees C warming – or 0.8 degrees C cooling – to an area; that’s a potential 1.6 degrees C:  twice as much as average global temperature rise since pre-industrial times.’

OK, so it turns out that it is more complicated than that, but at least now we know why….. All comments welcome, either on the blog, or direct to John at

October 14, 2009
Duncan Green