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.
The weather witnessed in Southeast US earlier this year was phenomenal. Over 600 tornadoes were recorded in April alone, breaking the previous record by several hundred. Numerous F5 tornadoes (the highest grade) with surface wind speeds greater than 200mph were also witnessed. The result was widespread and devastating destruction, including the deaths of more than 350 people. In Alabama, between 1950 and 2006, 358 people have been killed as a result of tornadoes. In the 30 day period of April 2011, 240 people were killed in Alabama.
Why so many tornadoes?
The high number of tornadoes was partly due to the Arctic Oscillation which was in a negative phase. High pressure in northern Canada, pushed the cold arctic air a long way south. On top of this, the sea surface temperatures in the Gulf of Mexico were a couple of degrees above where they should have been for the time of year. The resulting evaporation meant warm humid air residing off the southeast coast of the US. A southerly breeze, brought this warm humid air inland, and mixed with the cold air, creating a very active weather front. In this situation, the warm air rises up above the cold air and it begins to interact with the jet stream. The jet stream was further south than it should have been and this is partly due to the La Nina phase of the ENSO oscillation. As the warm air rises and meets the cold air, almost instantaneously as a thunderstorm develops, you get the twisting motion as a result of the cold and warm air meeting with the jet stream. The twisting motion produces the tornadoes which then eventually reach the surface. The image below shows the air movements and tornado reports across April 2011.
The daily evolving 500mb heights (contours) and 850mb wind (arrows) from April 1-30, 2011. Index: category 4 (orange) indicative of enhanced severe storm risk and category 5 (red) indicative of enhanced tornado risk. Source
A paper published in 2008 looked at whether tornado counts change location based on phase of El Nino/ Southern Oscillation. However, results found that neither frequency of tornado days nor days of violent tornados is affected systematically by the phase of ENSO for the US as a whole. Rather that ENSO only sets a ‘background stage’ for which tornado activity to occur in.
“The apparent response of organized tornado activity to ENSO phase is a nonlinear one driven by meteorological processes rather than conditions in the tropical Pacific. Neither ENSO extreme (warm nor cold phase) is related to as significant of an increase in organized tornado activity as the intermediate neutral phase is.”
Currently further research is required into the contribution of La Nina, and NOAA observational sources suggest that the tropical sea surface temperature conditions played a much larger role in what was witnessed in April 2011.
Anthropogenic Climate Change and Tornadoes
In order to assess long term climate trends, reliable long term data is required, which makes the assessment of tornadoes and climate change difficult. The US Climate Change Synthesis Report SAP 3.3 concludes that:
"The data used to examine changes in the frequency and severity of tornadoes and severe thunderstorms are inadequate to make definitive statements about actual changes."
"There were no significant changes in the high-intensity end of these distributions from the 1950s through the 1990s, although the distribution from 2000 and later may differ."
The historical record of tornado counts should always be treated with care. In some states, tornado counts have doubled in the last two decades, but this may be due to non-meteorological changes. Changes in use of equipment which result in large numbers of F0 (weakest) tornadoes being recoded which previously weren't, account for this rapid change in the recent totals (see records from Illinois below). Population growth and spotter networks also increase the amount of tornadoes witnessed and reported. Death tolls are also not necessarily that useful. In May 2004, 384 tornadoes were recorded, but only 7 deaths. In 2008, only 40 tornadoes are on record, but more than 100 people were killed. The high death toll years are often a result of just several tornadoes hitting urban centres with high populations.
a)
b)
The annual number of tornadoes per year in Illinois since 1950, regardless of strength (a) and F0 only (b). Source
"There is insufficient evidence to determine whether trends exist in.....small-scale phenomena such as tornadoes, hail, lightning and dust-storms."
Another paper on the analysis of climate change projections suggests that the number of days during which meteorological conditions are favourable for severe storms may increase during latter decades of the 21st Century, primarily due to increased instability as a result of anthropogenic forcing. However, the research projected decreases in vertical wind shear, which as a result may oppose thermodynamic destabilization. In such a case, there may not be a discernable link between tornados and climate change, which rely on increases in wind shear to form.
In summary, more research is clearly required to better understand how ENSO and its multi-year life-cycle may influence the probability of major, destructive tornado outbreaks over the US. The relation is likely to be more complicated than the simple state of the tropical Pacific sea surface temperatures.There is a large body of knowledge rapidly evolving over the possible role of large scale climate forcing caused as a result of anthropogenic impact, on tornado outbreaks. In order for this to be more successful, efforts need to me made in linking meso-scale meteorology with global-scale climate dynamics. Watch this space...
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.
