Is our climate getting more extreme?

Back in April 2013, snow fell over London and South East, with the Daily Mail calling the phenomenon the ‘Great Spring Freeze’. A week later, temperatures increased to balmy conditions of 13°C. Scrolling through the comments of that Daily Mail article, it was unsurprising to see that many readers had as usual been quick to claim that our weather is becoming more extreme and that these are events occurring out of the blue. However, are events similar to the ‘Great Spring Freeze’ really signs that our climate is indeed getting more extreme? 

In this first proper post of the blog, we shall look at the findings of three recent reports in order to answer the question: 

1. IPCC (2012) Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX)
2. WMO (2013) The Global Climate 2001-2010: Decade of Climate Extremes Summary Report
3. Trenbeth (2011) Changes in Precipitation with Climate Change

Before, we dive into the question proper, let us start by defining the term extreme. The IPCC (2012) report defines an extreme (weather or climate) event as ‘the occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends of a range of observed values of the variable’.

All the three reports have largely the same findings, although the IPCC seemed to be more careful in its reporting, qualifying each of its statements with a confidence level (e.g. very likely 90-100%, more likely than not 50-100%, unlikely 0-33%, very unlikely 0-10%). Some of the key findings of interest from the reports have been summarised below:  

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Temperature

Since 1950, it is very likely that the number of cold days and nights have decreased, while the number of warm days and nights has seen an overall increase (IPCC 2012). This finding corresponds with the WMO (2013) report that showed that the past decade (2001-2010) has been the warmest decade since modern meteorological records have been available around the year 1850. The global average near-surface temperature over the 10-year period is estimated to have been 14.47°C ± 0.1°C, which is 0.47°C ± 0.1°C above the 1961-1990 global average (Fig.1). In fact, 2010 was the warmest year on record.

Fig.1 Decadal global combined surface-air temperature and sea-surface temperature (°C). The horizontal grey line indicates the long-term average value for 1961-1990 (14°C). Source: WMO (2013)

Precipitation

As the climate warms, it is expected that the amount of precipitable water should increase as well since the Clasius-Clapeyron equation shows that the water-holding capacity of the atmosphere increases by 7% for a 1°C rise in temperature. This has translated to increases in heavier rains occurring in most places (Trenbeth 2011; IPCC 2012), with an increase in extreme flooding experienced in the 20^th century (Trenbeth 2011). It is also very likely that heavy precipitation events will increase in frequency over most areas of the globe in the 21st century according to the predications made by the IPCC (2012) (Fig.2). 

Fig.2 Projected annual and seasonal changes in three indices for daily precipitation for 2081-2100 with respect to 1980-1999, based on 17 GCMs contributing to the CMIP3. The left column shows the changes in wet-day intensity, while the right column shows the changes in percentage of days with precipitation above the 95% quantile of daily wet day precipitation for that day of the year, calculated from the 1961-1990 reference period. Source: IPCC (2012)

On the other hand, the Trenbeth (2011) report also mentioned that drought events have also increased over the 20^th century as measured by the Palmer drought severity index (PSDI) This is partly due the increase in surface temperature, which increases the atmospheric demand for moisture and hence an increase in evapotranspiration rates.

Nonetheless, both IPCC (2012) and Trenbeth (2011) concluded that while the character of precipitation has changed, the overall volume of precipitation has not increased.

Tropical cyclones and extratropical storms

Both the IPCC (2012) and WMO (2013) reports are not able to confidently say that there has been any clear trend found in the activities of tropical cyclones and extra tropical storms at the global level. However, in the WMO (2013) report, it is cited that the 2001-2010 period has been the most active decade since 1855 for tropical cyclones in the North Atlantic Basin. Meanwhile, there has likely been a poleward migration in the main Northern and Southern Hemisphere extra tropical storm tracks in the past 50 years (IPCC 2012).

Monsoon and ENSO

There are still many uncertainties involved in understanding monsoon regimes, however, there is a likely increase in extreme precipitation in monsoon regions (IPCC 2012). Nonetheless, this might not necessarily be due to changes in monsoon characteristics and not necessarily occurring in all monsoon regions.

As for ENSO, there is medium confidence in a recent trend toward more frequent central equatorial Pacific El Niño events, but there is insufficient evidence for more specific conclusions about the observed trends in ENSO (IPCC 2012). 

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Based on these findings, it seems like there has indeed been some changes in the trends of specific climate variables. Average global temperatures seem to be rising while precipitation appear to be getting heavier. However, it is difficult to confidently conclude that our climate is indeed becoming more extreme simply by looking at these trends. To do so, one would have to take into account the trends of all the different climate variables in order to make a comprehensive conclusion about the climate. However, thus far, all the metrics that have attempted to do so, including the Climate Extremes Index (CEI), are still not sufficient to take into consideration all the different climate variables. Moreover, one would also have to factor in all the feedback cycles involved within the climate system, which could lead to a damping or enhancement of the initial response to a given forcing.

While it may be difficult to give a definite answer on whether the climate is getting more extreme or not, it is nonetheless still possible to try and understand what are the causes behind the changes noted in the trends of the various climate variables mentioned above. Do anthropogenic activities contribute to these changes? Or are they within the natural variability of the climate? More importantly, what are the effects of these changes on natural disasters? What would an increase in sea surface temperatures have on future hurricane activities? What are the impacts of increased frequency of heavier precipitations on the flood risks of monsoon regions? These questions will be discussed in greater details in my upcoming posts over the next few weeks.