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Château Rouquette

December 2007: Climate Change and Bordeaux Revisited

Two Years Ago….

We examined the temperature in Bordeaux since the War to see if there was evidence of the effect of global warming. What we found was more interesting than the conventional wisdom that temperatures had been getting warmer. We showed that in 1988-90 there was a sudden jump of 1.5ºC in average temperatures but since then there had been no further significant warming. Contrary to the global warming thesis, temperatures had stayed on this higher plateau for the past 15 years. These higher temperatures have been good for wine quality – accounting for as much as 15 points out of a hundred in the vintage scoring system, according to one estimate. Vintages have been more consistent with far fewer bad years. Where bad years have occurred this was mainly due to high rainfall around harvest – the early 1990s were hit especially hard in this respect – rather than cool summers. However, we suggested that if temperatures were to rise further this could begin to harm quality – the hot summer in 2003 was a warning of what could occur.

The Recent Evidence

We have updated the key chart we used two years ago, which shows the rolling 12 month temperature for Bordeaux since the War:

The jump in temperatures in 1988-90 is clear, as is the subsequent plateau. There is no indication that this pattern has shifted. However, with the benefit of two more years of data, there may be some slight evidence that the plateau has a very slight upward bias. Each of the last 3 peaks, in 1998, 2003 and 2007 has been 0.1-0.2ºC warmer than the previous. The troughs also show an upward trend. Whilst this trend may be consistent with the global warming thesis, it is dominated by the big jump in temperature that occurred in 1998-90.

Of course, probably the most important temperature statistic is the temperature during the growing season (April- September). It would be possible that hotter summers and colder winters hide a rise in the growing season temperature. However, there is no evidence that this is has happened:

2003 was exceptional in that a hot year was compounded by a summer that was unusually hot relative to the average temperature for the year. Normally the growing season is 4.2ºC warmer than the average yearly temperature. In 2003 it was exceptionally 5.3ºC hotter and this entirely accounts for the spike in the chart.

2006 has been heralded as a “cold year” but this is not really true, as can be seen from the chart. Even if the hot April is excluded the growing season was no colder than in 2002. It has simply followed four much hotter years.

In summary, the temperature patterns of the last two years do nothing to invalidate the conclusions from our newsletter two years ago. Temperatures remain fixed within the higher plateau established in 1998-90. Cabernet Sauvignon normally needs a growing season of at least 16.5ºC to mature. Prior to 1988, 36% of the years saw temperatures below this resulting in generally unripe, green wines especially if Cabernet based. Since 1988, every year has been above 16.5ºC and there have been few, if any, unripe vintages.

Future Quality: an End to the “Dream” Years?

The two climatic keys to great vintages are warm growing seasons and lack of rain at harvest time. Very often in the past, these have not coincided. The 1990s were the classic example of this, with the benefit of high temperatures masked by generally high rainfall:

The early 2000s in contrast, have seen the dream combination of high temperatures and low rainfall, a combination that was last seen in the late 1980s.

The chart above shows the 5 year average rainfall (in pink). It can be seen that over the past 60 years there has been a cycle of about 8-15 years in rainfall. Based on this cycle, it appears that we are ending a period of dry weather and can expect about 5-7 years of wetter conditions. 2007 was possibly the first of these years, as rainfall returned to historically average levels. On the basis of this history, we can expect increasing levels of rain over the next few years. Of course, high rainfall during the year does not necessarily mean high rainfall during the short harvest period, but it tips the odds in favour of this happening. Furthermore, whilst all areas in Bordeaux have similar temperatures, rainfall can vary significantly in the short term causing big differences between different appellations.

Nevertheless, there is a risk that the quality of the next few vintages of Bordeaux will deteriorate due to higher rainfall. The early 2000s may be increasingly viewed as a “dream period” when warm temperatures and low rainfall coincided.

Michael Banton         5 December 2007