A final thought

Over the past weeks, we have witnessed extreme weather events occurring in the present, examined how extreme event trends may alter in the future, and how they have appeared to change from past to present. When it comes to predicting the future, however, I hope I emphasised how uncertain these projections and speculations are: I found this article on the BBC News just today questioning current climate model data..

http://www.bbc.co.uk/news/science-environment-20947224

Saying this, there is the general consensus within the scientific community that events such as hurricanes and coastal flooding are to increase in some regions in the future. Predictions can only be as good as current model data, and at the moment it is agreed we are living in a warming world. This will lead to rising sea levels, higher levels of moisture uptake into the atmosphere and warmer oceans.
Unfortunately, all of these factors add to an increased chance of hurricane formation and flooding...so keep hold of your hats!!

And now a look back in time...

Looking at past global climate helps scientists to gain an overall picture of climate variability; studying past events aids in discovering whether present day climate is becoming more unpredictable, or whether it is part of a longer trend. This study carried out by Zheng et al. (2006) aims to reconstruct the climate of eastern China over the past 1500 years using a combination of different proxies. In order to do this, a new dataset was constructed out of 3 sets of historical data: this is not ideal as different sourcing techniques were used and so the dataset could contain error or inconsistences (it is noted that there are many missing records before 1470).

In order to overcome this problem, Zheng et al. produced a Dry-Wet Index which analyses the long term change in precipitation instead of analysing individual records. This would help to decrease the error within the dataset and to focus on climate variability trends. The study area was divided into 3 regions based on their climatic differences, so that the natural occurrence of precipitation did not impact too much on the study; the aim was to find the variation in precipitation so it had to be accounted for that some regions were naturally wetter than others.

Of course, it was found that the dry-wet cycle within eastern China varied considerably over the past 1500 years (please refer to the article). But what about what we’re really interested in here, extreme weather?

The results from the study indicated that 18 flood events occurred within eastern China between 501-2000, with a 4 year flooding event occurring 1422-1425. Three major wet epochs were identified 880s-990s, 1240s-1420s and 1540s-1910s. Of course, there was variation in the climate within these times too. The occurrence of flooding has continued into the last 300 years, although the number of extreme droughts has decreased. Perhaps this is a sign of the changing climate? It has been said that the monsoon cycle is intensifying, could it perhaps be possible that these records are reflecting this?

By looking at the figure above, taken from Zheng et al., it can clearly be seen that the frequency and severity of droughts (indicated by the red bar) is decreasing as the records near the present day. The flood occurrences (blue), however, appear to becoming more frequent and are just as severe (indicated by the bar width) as those occurring in the past 1500 years. Although there has to be some allowances made for the dataset used, with better records obviously available towards modern day, the general trend does infer that eastern China is generally becoming more vulnerable to extreme flood events.

In my opinion, these results concur with recent research that the hydrological cycle is intensifying in light of global warming (Duracket al. (2012)).  This will likely increase the intensity of the monsoon (Ruosteenoja 2003) as with a warmer atmosphere which has a greater ability to absorb moisture, and higher evaporation rates occurring from warmer ocean surface waters (Cubasch et al. 2001) there is greater potential for precipitation. Currently the monsoonal rains occur from June to September, but in a warmer world this window could widen. Add to this the fact that sea levels are rising due to increased melt water and thermal expansion (Cubasch et al. 2001) and it is easy to see how the likelihood of extreme flood events seems to be increasing. Of course, it is extremely difficult to predict how our climate will alter in the future, but at the moment, the trends are all heading in the same direction…

The future of the hydrological cycle

'A change to freshwater availability in response to climate change poses a more important risk to human societies and ecosystems than warming alone. Changes to the global water cycle and the corresponding redistribution of rainfall will affect food availability, stability, access, and utilization.'

You may think that looking at the hydrological cycle in the future is going a little off topic when it comes to extreme weather events; but a rise in ocean temperatures could offer a greater possibility of hurricane formation over surface waters, and a rise in sea level creates a higher level of vulnerability for coastal populations. It is all connected.

I've linked a recent article by Durack et al. to this post, which caused quite a stir when it was published in Science (2012), as it proposed that for every 1°C warming in surface temperature, the hydrological cycle could intensify by 8(+/-5)%. This means that a 2°C warming in the future, (which isn't unlikely at the current rate of GHG consumption) could see a 24% intensification of the hydrological cycle. This would affect the entire globe, with current wet regions receiving more precipitation and those regions that are drier experiencing more frequent drought.These results were generated by Durack et al. by running sea surface salinity (SSS) data with current climate estimates (CMIP3 scenarios). In my opinion, SSS is a sensible measure to use as it correlates highly with evaporation-precipitation patterns and is far more reliable than the precipitation records; which offer scarce coverage over the oceans and may contain error in gathering techniques (e.g. satellite readings). The higher the salt content observed, the higher evaporation rates.
Of course studies such as this one have been conducted in the past, but the recent CMIP3 scenario data that this model ran with offers a more accurate view of global climate dynamics than has been used previously. Other models ran with more simplistic datasets: as we all know the climate system is very complicated, so leaving out one small factor can impact heavily on results. Of course, the CMIP3 scenarios aren't perfect, but they are the best we have.

The purpose of me drawing your attention to this article is the fact that it is possible that the 1°C of warming that we have experienced over the last century is having a greater impact than we imagine. Although the findings of this study are quite controversial, and in some cases are the complete opposite of other studies; I think that this experiment has ground to stand on. The reasoning behind using SSS as a representative factor of E-P is sensible and the scenario this data was run with is the best that is available. Durack infers that during the last 50 years, the water cycle has amplified by 4%: with another 1°C warming, it could be amplified by a further 8%. This would greatly increase the likelihood of flooding and hurricane formation.

So, as I said, this may have been a little off topic, but I hope you can see how it is an important in relation to the subject. I think Durack et al.'s paper is insightful and although it is definitely not the only work on the subject out there, I hope you can see how it makes a strong case. Anyhow, it's a nice read and also makes you think about how our actions are influencing tomorrow's world, so enjoy... 

Its raining again

Welcome back, hope you enjoyed the festive season. As the video on the previous post highlighted, there has been a recent increase in flooding (amongst other extreme events) in the UK. After having studied Robson's article 'Evidence for trends in UK flooding' I will decide whether or not it is likely that these events have been caused by climate change.


A scene from a small village in York after the flooding in February 2010- could the UK see more of this as the climate changes?

Robson’s 2002 study of trends in UK flooding emphasises how difficult it is to distinguish whether or not climate change is impacting on our weather conditions. Although some trends have indeed shown that winter rainfall and number of high flow days is increasing, this can not necessarily be attributed to climate change. Instead it can be attributed to natural climate variability, which can see short term changes in local weather conditions; but when trends are averaged over time, the average climate conditions have remained in a stable state. Unfortunately, although Robson did find evidence of increasing rainfall rates, the data series in the UK is not long enough (~60 years of appropriate coverage), nor reliable enough to rule out the cause being climate variability. Other factors such as changing land use can affect flooding records, especially when the changes involve urbanisation or agriculture as these can impact greatly on infiltration rates. This is why an urban environment is at greater risk of flooding.

So, although Robson did find evidence of increasing rainfall and number of high flow days in the UK, with the results from Scotland closely echoing the trends found in previous studies; climate change cannot be assigned as the cause, as other factors can’t be ruled out. In my opinion, however, with similar events happening around the globe; it is more likely that the main driver is the climate, whether it is just varying in the short term, or changing in the long term. For me, it is preferable to note this as the cause rather than change in landuse, although it is noted that this does play a role. Either way, due to this, the UK is experiencing more extreme weather events today than it has done in the past.