2011 - A Year of Extremes, and More To Come...

Firstly, I hope you all had a great Christmas! It is this time of year that makes you really grateful for just the basic things in life. We are now fast approaching 2012 and is a good time to reflect on the past 12 months.

I don't think anyone can doubt that 2011 has had it's fair share of extreme weather events. December has seen the devastating monsoon in the Philippines that killed over 700, Scotland battered by a storm with wind speeds in excess of 165mph, whilst flooding in Kenya displaced thousands. November saw a mounting death toll in Cambodia after thousands were left stranded after monsoon conditions. In October residents fled Bangkok as flooding threatened to overwhelm the capital at the time when Texas was facing a dust cloud 8,000ft high, and the UK was facing it's highest October temperatures on record.  There was also the freak snow storm along the US east coast which killed 11 and left millions without power, only two weeks after hurricane Irene had ripped through the Caribbean and the south-east. September had seen even more destruction for Texas in which huge wildfires destroyed hundreds of homes. Going back to May, the world witnessed one of the most extraordinary tornado sequences ever caught on camera with more than 165 in just 24 hours in the Missouri area. March saw the most devastating natural event of the year in which Japan was hit by a double tsunami leading to 20,000 deaths and millions of pounds of damage. Cyclone Yasi has long been forgotten, but struck the Australian coast only back in February, tearing roofs of houses and bringing power lines to the ground. Going right back to the beginning of the year, January saw 400 killed as a result of torrential rain and mudslides in Brazil, there was flooding in Sri Lanka which forced 300,000 from their homes, as well as flash floods in Queensland.

Climate change sceptics may suggest that we are just becoming more 'aware' of extreme events since the 'global warming' hype of the 1980s but from research over the past few months, and the experience of extreme weather events in my lifetime I can only come to the one conclusion that anthropogenic climate change is a primary driving factor in the increasing frequency of these extreme events.

An extract from the Scientific American, dating back to June, summarises this nicely through the results of one of the worlds largest insurance companies Munich Re. The company has compiled the world's most comprehensive database of natural disasters, reaching all the way back to the eruption of Mount Vesuvius in A.D. 79. The records are kept with extreme interest by financial companies, in order to successfully evaluate insurance risks. On average 700-1000 natural disasters are added to the database each year. Their data shows a small increase in geologic events such as earthquakes since the 1980s, largely down to better reporting. However, increases in the number of climate disasters is 'far larger'. 

Peter Höppe, head of Munich Re's Geo Risks Research/Corporate Climate Centre:

 "Our figures indicate a trend towards an increase in extreme weather events that can only be fully explained by climate change... It's as if the weather machine had changed up a gear."

In the latest SREX report, we have seen the IPCC state:

"There is evidence that some extremes have changed as a result of anthropogenic influences, including increases in atmospheric concentrations of greenhouse gases."

Direct cause and effect literature has been hard to come by in the past, but this year has seen a significant increase not only in literature suggesting increasing likeliness of climate change on extreme events, but also as direct attribution. This does not mean however that every extreme event is a direct result of anthropogenic climate change. For example, solar forcing causing winter lows which I looked at in a previous post, can be assigned to natural variability. There has always been extreme events, and always will be as a result of natural variability, but the latest findings point to both an intensification and frequency of such events as a result of human impact. Over time with better climate model simulations, greater understanding of the hydrological cycle (specifically with vorticity and wind speed), and an increase in data collaboration and research papers, will I'm sure, only pile on greater evidence that extreme weather is slowly becoming the new normal (unless human anthropogenic impact is significantly minimised or reduced).

All I can conclude with is a warning: Extreme Weather Ahead.

Signing off for 2011.

IPCC Special Report - Latest Findings on Extreme Weather Events

The report suggests that droughts will intensify during the 21st century

Working Groups I and II for the IPCC have produced a special report for 'Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation' (SREX for short). The findings of the Special Report were presented at the IPCC's 34th session, which is currently being held in Kampala, Uganda. The full report is not available until February 2012, but a 29 page summary has been made available and can be downloaded here.

I've listed some of the key findings below, but I would urge you to take a look at the full report yourselves:

> "A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events."

> There is evidence that some extremes have changed as a result of anthropogenic influences, including increases in atmospheric concentrations of greenhouse gases. It is likely that anthropogenic influences have led to warming of extreme daily minimum and maximum temperatures on the global scale.

> "It is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy falls will increase in the 21st century over many areas of the globe."

> "Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged."

> "There is medium confidence that droughts will intensify in the 21st century in some seasons and areas, due to reduced precipitation and/or increased evapotranspiration."

> There is high confidence that changes in heat waves, glacial retreat and/or permafrost degradation will affect high mountain phenomena such as slope instabilities, movements of mass, and glacial lake outburst floods.

The summary is very cautious with it's findings and confidence levels, stating that "Extreme events are rare, which means there are few data available to make assessments regarding changes in their frequency or intensity." As has been discussed in previous posts and seen in the current academic literature, it becomes very difficult to isolate a single event, like a heatwave or a heavy rainstorm, and say that event was caused by the human element of climate change.

Despite this, the report is yet another contribution to the scientific community suggesting that anthropogenic impacts are having a direct impact on extreme weather events.

165 Tornadoes in 24 Hours

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

The more recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) goes on to state:

"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...