In February 2014, Nigel Lawson and Brian Hoskins (Chair of the Grantham Institute for Climate Change) appeared on the BBC’s Today show to answer whether there was “a link between the rain in recent days and global warming”.
Lawson, an experienced man of affairs though not a “climate scientist”, briefed himself on the matter and gave an answer was in accordance both with the findings of the most recent IPCC report and even with Hoskins’ own prior statements. In contrast, Hoskins, though an eminent climate scientist, gave a woolly response that quickly digressed into Green talking points.
Predictably, green activists complained both about Lawson’s answer and even his appearance on the show. The Today show rejected the initial complaints. However, green activists, including Bob Ward, who like Hoskins is supported by a Grantham institute, filed further complaints. In late June, the Guardian reported that a decision by Fraser Steel of the BBC Complaints Unit had issued a finding that Lawson’s views were “not supported by the evidence from computer modelling and scientific research”:
Lord Lawson’s views are not supported by the evidence from computer modelling and scientific research … and I don’t believe this was made sufficiently clear to the audience …
Steel went on to make other adverse findings against Lawson. However, in respect to the issue raised by the programme – the “link between the rain in recent days and global warming” – Lawson’s views were supported by scientific research, while Hoskins evaded a direct answer, instead quickly digressing into green talking points not directly to Somerset rainfall.
In today’s post, I’ll examine the answers of both Lawson and Hoskins against IPCC statements and, ironically, against Hoskins own prior statements (which are inconsistent with the complaints.)
In January 2014, the UK had experienced intense precipitation – 185.1 mm, ranking in the top percentile of UK rainfall months – 16th in the historic series reaching back to 1766, but nonetheless ranking behind four 18th century and three 19th century months. The rainfall was particularly severe in the UK southwest.
Justin Webb, the BBC presenter, commenced the program by asking Hoskins, whether there was a “link” to global warming:
Is there a link, Sir Brian, between the rain we have seen falling in recent days and global warming?
Both Hoskins and Lawson gave answers to this question before digressing to their respective talking points.
IPCC on Heavy Precipitation
IPCC AR5, consistent with earlier reports, projects that increased temperatures will result in more water vapor in the atmosphere. Based on the Clausius-Clapeyron rate, they estimated a global increase in precipitation of ~7% per deg C., with the base case for increase in precipitation extremes also being ~7% deg C. (For an observed increase of ~0.8 deg C, this would be ~5.6% increase in precipitation extremes.) IPCC:
Trenberth et al. (2003) provided a physical explanation for why increasing atmospheric temperature might result in an increase in heavy precipitation and suggested that extreme precipitation should scale with the water content of the atmosphere (see also Allen and Ingram 2002). The water content has been found to scale roughly at the Clausius-Clapeyron rate of ~7% K^-1 based on both observational and modeling studies, with the possible exception of the drier land regions, where the scaling appears to be lower (O’Gorman and Muller 2010; Sherwood et al. 2010a; Simmons et al. 2010; Willett et al. 2007). Therefore, based on this hypothesis, one would expect annual maximum daily precipitation to increase in most regions globally at a rate of ~7% K^-1.
While the IPCC anticipated an overall global increase of ~7% per deg C, it definitely did not expect such increase to be uniform: indeed, IPCC presumes that precipitation in some regions may not increase at all.
In its Figure 7.21, IPCC showed GCM estimates of increased precipitation as much lower than the CLausius-Clapeyron rate (visually about 1-3% per deg C). These figures were not reported in the text. Instead, IPCC said that IPCC observed that GCMs were “generally poor” at simulating precipitation extremes and “are not usually thought of as a “source of reliable information regarding extremes.”
Because GCMs are generally poor at simulating precipitation extremes (Stephens et al., 2010) and predicted changes in a warmer climate vary (Kharin et al., 2007; Sugiyama et al., 2010), they are not usually thought of as a source of reliable information regarding extremes.
Further, the IPCC’s section on paleoclimate reported that there was convincing proxy evidence that 20th century floods were not only not anomalous, but were easily surpassed by historic floods, with higher flood frequency in the UK in cool phases:
Reconstruction of past flooding from sedimentary, botanical and historical records (Brázdil et al., 2006; Baker, 2008; Brázdil et al., 2012) provides a means to compare recent large, rare floods, and to analyse links between flooding and climate variability. During the last few millennia, flood records reveal strong decadal to secular variability and non-stationarity in flood frequency and clustering of paleofloods, which varied among regions. In Europe, modern flood magnitudes are not unusual within the context of the last 1000 years (e. g., Brázdil et al., 2012). .. In the Alps, paleoflood records derived from lake sediments have shown a higher flood frequency during cool and/or wet phases (Stewart et al., 2011; Giguet-Covex et al., 2012; Wilhelm et al., 2012), a feature also found in Central Europe (Starkel et al., 2006) and the British Isles (Macklin et al., 2012).
In summary, there is high confidence that past floods larger than recorded since the 20th century have occurred during the past 500 years in northern and central Europe, western Mediterranean region, and eastern Asia.
The conclusion of AR5 chapter 2 (observations) reported a similar conclusion:
there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale
Macklin et al 2012, cited above by IPCC for the UK, stated explicitly that changes in flooding regimes evidenced from (recent) paleoclimate records in the UK were greater than the instrumental record, with flooding greater in the cold LIA:
the ﬂoodplain sedimentary archive is compared with long-term proxy NAO records [108- Trouet et al 2009], which shows a marked reduction in the occurrence of large ﬂoods during the MCA, a time of generally warmer temperatures and a more positive NAO, compared with the period before AD 1000 and particularly after AD 1550 during the cooler LIA (ﬁgure 10b). These studies demonstrate repeated and signiﬁcant changes in ﬂooding regime in the last 500–1000 years, which were very much greater than those that have been observed in recent instrumental ﬂow records
Macklin and Lewin 2008 (EPSL) estimated UK flooding regimes through the Holocene, with their Figure 6 showing especially high flooding incidence in the LIA.
The IPCC also considered the specific question of tropical storms(hurricanes), where it conceded (resiling from AR4) that observations did not indicate a long-term upward trend:
Over periods of a century or more, evidence suggests slight decreases in the frequency of tropical cyclones making landfall in the North Atlantic and the South Pacific, once uncertainties in observing methods have been considered. Little evidence exists of any longer-term trend in other ocean basins.
IPCC chapter 2 (observations) considered heavy precipitation only in the context of the past 50 years, where it stated:
Regional trends in precipitation extremes since the middle of the 20th century are varied (Table 2.13). In most continents confidence in trends is not higher than medium except in North America and Central America and Europe where there have been likely increases in either the frequency or intensity of heavy precipitation.
UK January 2014 rainfall in Context
UK concern over heavy January 2014 precipitation arose largely because of flooding in Somerset (see e.g. here). At the time, there was considerable discussion about the degree to which changes in floodplain management practices had made the region more vulnerable to heavy (but precedented) rainfall, as opposed to vulnerablity arising from supposedly unprecedented rainfall.