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.)

The Question in the Today Programme

The Today interview in controversy is available in transcript here and audio here.

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.

The UK has a long dataset of historic precipitation measurements: their England and Wales series goes back to 1766 i.e. commencing after the LIA extreme in the UK.

Precipitation in the UK occurs throughout the year, heavier in the late fall and winter than in the summer. Median monthly precipitation is 72.8 mm, but is highly variable: the 2.5-97.5% quantile spread of 138 mm. January 2014 rainfall was in the 99th percentile but not unprecedented. The England and Wales series (shown below) is not HS-shaped as shown below:



Figure 1. England and Wales Monthly Precipitation Anomaly (1766-2014). 61-month gaussian smooth in red.

Hoskins 2012

In 2012, Hoskins (see here), in response to questions about links between heavy precipitation and global warming, had explicitly stated that asserting such links “goes beyond” what can be said with confidence, though it “does a great job of keeping climate change in the news”:

I know that some scientists, like high profile NASA climate expert James Hansen, already say there is a definitive link between the occurrence of the extreme events and increase of greenhouse gases in the atmosphere. This does a great job of keeping climate change in the news, but to me it goes beyond what we can say with confidence based on current scientific understanding and evidence.

He observed that one of his students had been trying to examine projections of extreme storms, reaching an agnostic conclusion. Hoskins ironically observed that perhaps the media who claimed that we were going to get “more and stronger storms” knew “more about it than [he] did”:

One of my PhD students, Erica Thompson, has been looking into how extreme storms might change in the future but she has concluded that the models and records we currently have aren’t good enough to make a confident prediction. You often hear in the media that we’re going to get more and stronger storms, but maybe they know more than I do about it!

Hoskins observed that many periods of extreme weather in UK latitudes came from “blocking highs”, observing that the cold UK winter of 2010 was accompanied by relative warmth in Greenland.

Many of these prolonged periods of extreme weather in our latitudes occur because of an atmospheric pattern called a blocking high. Regions get stuck under the same weather system for entire seasons and this interrupts the region’s normal weather patterns….All the events we’ve seen recently have happened before; the question is whether we have altered the climate to the extent that they’re occurring more often or with greater intensity.

“Marking” the Answers

After this brief review of IPCC positions on heavy precipitation and flooding, let’s now examine answers by Hoskins and Lawson’s to the question from the Today program presenter.

Justin Webb, BBC: Is there a link, Sir Brian, between the rain we have seen falling in recent days and global warming? Sir Brian Hoskins: There’s no simple link – we can’t say yes or no this is climate change. However, there’s a number of reasons to think that such events are now more likely. One of those is that a warmer atmosphere that we have can contain more water vapour and so a storm can bring that water vapour out of the atmosphere and we’re seeing more heavy rainfall events around the world. We’ve certainly seen those here. Justin Webb: So it’s the heavy rainfall; it’s the severity of the event that points us in this direction? Sir Brian Hoskins: Well, in this event we’ve had severe rainfall but we’ve also had persistence, and that’s where I say we just don’t know whether the persistence of this event is due to climate change or not.

In his December 2012 interview at Imperial College, Hoskins’ position on persistence was apparently rather different. At that time, he had observed that blocking highs were a feature of mid-latitude circulation. IPCC AR4 had reported a decline in North Atlantic blocking events. This was cited in AR5, which concluded “there is low confidence in characterizing the global nature of any change in blocking.”

While the IPCC relied on the Clausius-Clapeyron rate as indicating a ~7% increase in precipitation (and precipitation extremes) per deg C increase in temperature, the IPCC also emphasized that there could be major differences between regions. Indeed, in respect to the UK, they cited paleoclimate evidence (see above) which linked high UK flooding to cool periods rather than warm periods.

But even if one applied the Clausius-Clapeyron rate itself to UK precipitation, the expected change in precipitation resulting from the observed increase in temperature of ~0.8 deg C is much smaller than internal variability of UK precipitation. For argument’s sake, applying the Clausius-Clapeyron rate to the observed temperature increas of ~0.8 deg C yields an expected increase of ~5.6% increase in monthly precipitation. Applied to UK precipitation, this would increase the median by approximately 4.1 mm (from 72.8 mm to 76.9 mm). For rare events (e.g. 185.1 mm), a ~5.6% increase in monthly precipitation would reduce the return time of a 185.1 mm event. Alternatively, a 5.6% increase would mean that an event that was formerly a 175 mm event was now a 185.1 mm event.

In my opinion, one can argue that climate change might have marginally exacerbated the problem arising from an uncommon (but precedented) blocking high, but it is not possible to claim that the blocking high and related high precipitation was “caused” by climate change – a position that that is surely entirely consistent with Hoskins’ December 2012 interview.

This was Lawson’s position as well. When invited to comment, Lawson stated:

Certainly it is not the case, of course, that this rainfall is due to global warming – the question is whether global warming has marginally exacerbated it. Nobody knows that.

The rest of the interview diverged away from links between floods and global warming to more general issues of extremes around the world, with Hoskins citing recent Australian temperature, Arctic sea ice, Greenland and West Antarctic ice sheets – all prominent green talking points, but not immediately relevant to precipitation and flooding in Somerset.

Bob Ward’s Complaint

Bob Ward, also from a Grantham institute, published a lengthy complaint taking particular umbrage at Lawson’s (IMO reasonable) position that the salient issue in respect of flooding and global warming was “marginal exacerbation”:

Lord Lawson was obviously determined to dispute the science during the interview, despite his lack of expertise, and made a number of false claims which were not challenged or corrected by Justin Webb, the presenter who was carrying out the interview. For instance, Lord Lawson contradicted the scientific explanation for the link between climate change and the floods provided by Professor Hoskins, stating: “I think that Sir Brian is right on a number of points. He’s right first of all that nobody knows. Certainly it is not the case of course that this rainfall is due to global warming. The question is whether global warming has marginally exacerbated it. And nobody knows that.” In fact Professor Hoskins had laid out the scientific case very carefully, pointing out that it is not yet clear to what extent climate change may have contributed to the specific bouts of extreme weather that have occurred since late December, but also noting that the increase in intense rainfall that the UK is experiencing is likely to be the result of the warming of the atmosphere.

However, Hoskins’ remarks to the Today show were hardly the “careful” laying out of “the scientific case” claimed by Ward. In his written comments in December 2012, Hoskins took the entirely opposite position, saying that such claims did a “great job of keeping climate change in the news” but went beyond “what we can say”:

I know that some scientists, like high profile NASA climate expert James Hansen, already say there is a definitive link between the occurrence of the extreme events and increase of greenhouse gases in the atmosphere. This does a great job of keeping climate change in the news, but to me it goes beyond what we can say with confidence based on current scientific understanding and evidence.

The BBC Complaints Unit Decision

On June 25, 2014, the Guardian published that Fraser Steel of the BBC Complaints Unit had written to complainant Chit Chong, a Green Party politician, stating that “Lord Lawson’s views are not supported by the evidence from computer modelling and scientific research”.

In respect to the linkage between the floods and global warming, Fraser Steel’s views are unequivocally wrong. Even IPCC – surely the most fervent advocate of climate models imaginable – stated that GCMs did not provide useful information on precipitation extremes (and, a fortiori, floods). (Nor do the GCMs contradict Lawson since, as noted above, they show a lower than Clausius-Clapeyron increase in precipitation per deg C.) Nor does the Clausius-Clapeyron rate (referenced by IPCC) contradict Lawson, since, as noted above, it yields only a ~5.6% increase in precipitation for the 0.8 deg C increase in temperature – an increase that is much smaller than internal variability – entirely consistent with Lawson’s statement that the issue was one of “marginal exacerbation”.

Conclusion

I have not parsed other issues in the interview: Australian temperatures, global heavy precipitation extremes, Arctic sea ice, as each involves its own issues, and because the focus of the interview was on the linkage of heavy January 2014 precipitation and floods to global warming.

But clearly, inspired at least in part by Hoskins’ fellow Grantham Institute employee Bob Ward, the BBC has arrived at a factually incorrect and unfair decision in respect to the complaint against Nigel Lawson. Perhaps the person best placed to remedy the situation is Hoskins himself. Hoskins surely knows that Lawson was correct in his statement about the linkage between the floods and global warming ( the issue is “marginal exacerbation”). And in his statement about tropical cyclones. And about Chinese emissions. And that he has a legitimate argument on wind turbines.

If Hoskins and the Grantham institutes want to persuade more people of the seriousness of the issues, Hoskins’ obligation is to do a better job, rather than have Lawson silenced by a Grantham apparatchik. I think that Hoskins should write to the BBC Complaints Unit, separating himself from Ward’s complaint and, at a minimum, conceding that Lawson’s position on the (lack of) linkage of floods and global warming is either correct or one that can be reasonably argued.

It is, of course, vanishingly unlikely that Hoskins would do anything so gracious. Hoskins was the go-to person for the University of East Anglia when the Royal Society laundered the list of articles for the Oxburgh inquiry: although Hoskins himself had no informed knowledge of the literature, he immediately endorsed the UEA. Later, he acted as a supporting authority for refusing FOI requests.



