Climate Change and Cyclone Damage

Satellite Image of Cyclone Yasi from Feb 2011 (Image Source)

A very belated Happy New Year - it's been a while since I last blogged! Earlier today I was kindly pointed in the direction of a new paper which was released in Nature this week (the article can be found here).

The paper ties in nicely with some of the latest cyclone research that I blogged about in December. The purpose of the research was to assess future damages caused by cyclones and climate change. They are clear in their introduction that future scenarios with cyclone models are sketchy at best and that despite increases in cyclone numbers in the past 40 years, they are still well within natural variability. The paper is also quick to note that increasing damages in the last few decades are largely down to the increasing population and more assets in harms way rather than as a result of anthropogenic warming. However, they have taken the scientific viewpoint further by modelling cyclones in separate ocean basins, taking into account frequency, intensity and regional location changes (rather than just wind speeds and sea surface temperatures as seen in previous studies and models). The figure below shows the global storm tracks and their intensity (minimum pressure). It clearly indicates that the storms are more frequent over the warmer equatorial waters, and most occur in the Western Pacific.

The present annual global damage from tropical cyclones is US$26 billion and is used as a baseline for scenario comparisons up to 2100. The results in this paper, reveal (as would be expected) that the distribution of climate-change damage is not even across the world. The figure below shows the estimated projected damage caused by climate change in each region. North America has the highest average damage of US$26 billion yr-1, which is half of the global damage. East Asia and Central America Caribbean average damages of US$15 and US$10 billion yr-1 respectively.

There are some limitations with this study and its projections, which largely revolve around economics and countries GDP and growth rate. Damages vary a great deal if a storm hits a city versus a rural area, but this is not yet captured in this analysis. Moreover, increases in income and population along the coast, relative to the rest of the country, will cause more damage. There is also no account of any adaptation policy that could be put in place. Despite these limitations it is a nice to see an initial attempt at quantifying costs of cyclone damage associated with climate change.

Floods in England Caused by Anthropogenic Greenhouse Gases

Scenes such as those in Uckfield in the above video were witnessed all across England and Wales in October and November 2000. It had been the wettest autumn since records began in 1766 - the map below outlines total rainfall and percentage increase above average for the time of year. Over 10,000 properties were flooded and the costs as a result of the damage exceeded £1bn.

Image Source

In a recent paper published in Science, a group of scientists have been attributing flood risk and anthropogenic climate change as a result of the extreme events in 2000. As mentioned in previous posts, only general explanations are usually offered for any expected increase in flooding. They usually revolve around thermodynamic arguments for precipitation extremes, an example of which can be found here.

In this recent paper a seasonal-forecast-resolution climate model was used to create two scenarios. One realistic scenario whereby actual twentieth century climate was modelled (including atmospheric pollutant concentrations) and a second hypothetical scenario to represent climatic conditions as they might have been had anthropogenic greenhouse gas emissions not occurred. Atmosphere-ocean linkages were taken into account alongside sea surface temperatures and sea ice cover. The scenarios could then be run multiple times and runoff rates could be compared between natural and anthropogenic conditions.

The study outlines that there were sequences of intense weather systems bringing heavy precipitation pulses to catchments multiple times throughout the day in Autumn 2000. This was characterised by the displacement of the North Atlantic jet stream (see image below) bringing more intense systems further into western Europe. 

This resulted in many catchments becoming saturated. Daily river runoff was examined using a simple hydrological runoff model and the climate scenarios were compared.

Image Source

The above diagram is a summary of their results, showing that the fraction of risk attributable to twentieth century greenhouse gases are significant relative to the natural conditions (without anthropogenic impact). The black line is an aggregate relative to the full natural baseline conditions.

Model Results:

In nine out of ten cases, the model results indicate that twentieth century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales by more than 20%, and in two out of three cases by more than 90%.

These results are yet another step towards providing evidence of how extreme weather events are a direct result of human anthropogenic change. But again I will give a word of caution - changes in built environment (concrete over natural environments) could have played a part in historical model simulations in recent years (affecting both run-off and return time). Furthermore, just because an event-type becomes more likely, does not meant that it will become even more likely in the future - but it does highlight the potential impact of climate change..

Where's the proof?

Tornado makes it way through Baca County, Colorado, May 2010. Source

In the past year the globe has witnessed devastating drought in Africa, unusual tornado clusters in south-eastern USA, and catastrophic floods in Japan and Pakistan. It has always been difficult for climate scientists to provide proof for a direct cause and effect relationship between climate change and these extreme weather events. Predictions can be made how a warming climate can affect evaporation rates over oceans and then try to create links to cloud formation, water vapour and the atmospheric cycle in order to estimate frequency of these extreme events. But it is far from an exact science.

In the past two years groups of researchers from the United States and Britain have forme a loose coalition under the name ‘ACE’ which stands for ‘Attribution of Climate-Related Events’. They are in the process of laying out plans  for a near-real-time attribution system which can assess the changing climate’s influence on weather events as soon as they happen. The paper is being presented at the World Climate Research Programme conference in Denver, Colorado at the end of next week. It will be interesting to hear the scientific communities response to this.

However, one of the major difficulties in the attribution by ACE, and why there are still sceptics about attribution techniques, is the fact that freak weather cycles such as El Niño events have been occurring long before any anthropogenic impact. Each extreme weather event will subsequently have to be analysed as to how much was influenced by anthropogenic greenhouse warming and how much by natural cycles (baseline conditions). The analysis is also dependent upon the type of extreme weather. Temperature based events such as heat waves are far more simplistic and easier to model, but for precipitation events such as droughts and floods , the models don’t just have to take into account precipitation, but soils, natural terrain and human management of rivers and wetlands. Moreover, tornadoes are caused by a balance between moist air convection and wind shear, but it is very difficult to say for certain how climate change affects this balance.

The figure below from an article in nature summarises how attribution research attempts to quantify the effects and frequency of extreme events.

The costs of extreme weather events are substantial – both in terms of livelihoods and economic costs. The death toll from the Japan tsunami in March passed 18,000, and some 40,000 people died as a result of the heat wave across Europe in 2003. Economic costs on top of the death told make a bad situation even worse. The World Bank has estimated that it could cost Japan as much as £145bn to repair the damage from the 2011 tsunami.

As a result the global community should be ever more concerned about whether anthropogenic changes are the direct cause of these extreme events. At a very basic level you could say that humans are killing thousands of people as a result of anthropogenic warming. If these climate models and projects as put forward by ACE can help provide proof of direct links, they may help to provide some form mitigation by outlining the direct quantitative impact humans are having. It could also lead to a greater willingness for the global community to reduce greenhouse gas emissions, and give greater weight to policy decisions on the national and global scale.