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.

Monsoons and Climate Change

Image Source

After the devastating effects of Tropical storm Washi in the Phillipines earlier this week, I thought I would look further into any links between monsoons and anthropogenic climate change. It is well acknowledged that land use changes and human impacts through mining and illegal logging were a major cause in the flash floods, but 12 hours of continuous rain, in an area of the Philippines which storm tracks do not normally take is somewhat unusual. In an earlier post where I stated some of the latest finding form SREX, the tropical cyclone statement was one that particularly shocked me:

"It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged."

As a result I have done some digging to try and find other sources to these claims. I have come across two papers: one which looks at the evolution of Monsoons through the Holocene (12,000 years before present to modern day) using climate modelling, and another which assesses monsoon precipitation changes over the past 100 years.

The first paper deals with changes on the millennial time-scale and focuses on insolation forcing and feedbacks. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of ocean - atmosphere interaction. In the Northern Hemisphere, 'the models show a significant enhancement of all of the monsoons in the early Holocene and a gradual weakening toward the present'. The monsoons are enhanced in the Holocene by a positive oceanic feedback in North Africa and North America but are suppressed by a negative overall feedback in Asia.

The more recent second paper (July 2011) deals with changes on the decadal time-scale, over the past 100 years. Changes of global land monsoon precipitation are assessed by using three sets of rain-gauge precipitation data for the period of 1901-2002. It has been found that during 1901–2001, global land monsoon precipitation exhibits multi-decadal variations, with an overall increasing trend from 1901 to 1955, followed by a decreasing trend up to 2001. See the image below for the regional trends and Northern Hemisphere (NH) mean. There is uncertainty over changes in the 2000s.

Time series of precipitation anomalies for Northern Hemisphere land monsoon mean. (a) NH Mean (b) North African Monsoon (c) Indian Monsoon (d) East Asian Monsoon.

What does this all mean in brief?

The papers describe an overall weakening of monsoons on the millennial and decadal time scales, suggesting that anthropogenic warming of the past 200 years has not intensified them. I have looked in previous posts about how the intensification of the hydrological system can lead to more precipitation in some areas and decline in others, but I was not expecting such a significant downward trend of monsoons in the Northern Hemisphere over the past 40 years. It is a gentle reminder that ocean-atmosphere interactions are far more complex than we can imagine, and that despite SREX stating that heavy precipitation events will become more likely, it is unlikely that they will come through more extreme monsoon and tropical storm events. 

Moreover, it shows all the more clearly that land use changes can have such devastating effects on our planet, even without significant changes in weather patterns. The removal of the natural environmental buffer in the case of the Philippines has played a major role in the devastating effects and death toll, rather than an intensification of weather patterns.