It was my own fault, I admit it.

Speaking as someone who has lived through a sequence of canicules - heatwaves - in Bordeaux, I have felt and seen their impact.

In a post about the fires, I said I hoped that the people who intentionally or carelessly started them (95% are caused by man) get the punishment they deserved, but I also pointed the finger of blame at politicians who believe climate change to be a hoax and/or oppose net zero.

I should have known that this last comment would provoke responses from a group of people who include downright climate change deniers, 'sceptics', libertarians who really don't care but hate being told what to do by governments, and a few nostalgics for the days when LinkedIn posts were limited to money and business.

And I screwed it all up by implying that the people who don't think we should be doing our best to stop global warming might actually be doing all of our businesses quite a lot of harm.

Specifically, in the wine business, it can delay the grape ripening process, exacerbate the impact of wildfires and introduce the risks of smoke taint.

Setting aside the smoke taint, which I've written about separately, I think it's worth discussing the other factors.

VPD - a term you may hear more about

After a little research, I've learned about the VPD - Vapour-Pressure Deficit - that helps to explain why wildfires are often much bigger and faster-spreading than in the past.

Heatwaves cause disproportionate drying because the atmosphere’s capacity to draw water from soil and plants rises much faster than the temperature itself. It is not simply that 40°C is a third hotter than 30°C: the evaporative demand can be nearly twice as great.

The saturation vapour pressure of air increases by roughly 7% for every 1°C of warming at ordinary surface temperatures. Unless humidity rises equally quickly, the gap between how much water vapour the air contains and how much it could contain — the VPD - widens sharply.

For example, at the same 40% relative humidity:

• at 30°C, VPD is about 2.55 kPa (Kilopascals are the units used to measure the vapor pressure that drives and controls evaporation.)
• at 40°C, it is about 4.43 kPa.

This disproportionate effect means that a 10°C rise in temperature increases the atmosphere’s drying pull by approximately 74%, not 33%. And a few 37°C spikes will have a greater drying effect than a sequence of 30°C days with the same cumulative amount of heat.

Unable to sweat

For a wine region, increased VPD has several related consequences. First, water evaporating through a plant’s stomata cools the leaf, in the same way sweat cools skin.

During extreme heat, the vine initially transpires faster to maintain leaf temperature. But if the roots cannot replace that water quickly enough, the vine closes its stomata. This reduces water loss, but it also stops much of the evaporative cooling and restricts photosynthesis. The leaf can then become several degrees hotter than the surrounding air, intensifying heat injury, even though transpiration has slowed. In human terms, the vine is like a worker who has decided to down tools on a day when it's too hot to work. If there's work that has to be done, they'll do it later when everything is cooler.

No night shift

During a normal cool night: VPD declines; stomata and plant water status can recover; roots replenish water lost from leaves and shoots; respiration slows. The trouble with heatwaves like the ones we've been having is that temperatures may not drop sufficiently.

So, just as I've struggled to get to sleep, even with the help of a fan, the vines are continuing to dehydrate (unlike me, they haven't got a glass of water by their bedside). Stress accumulates from day to day rather than resetting each night.

The second problem is that dry soil gets even drier very quickly. This heats the vine canopy from below, increases the VPD immediately around the vine, accelerates evaporation from remaining damp patches, raises root-zone temperature and generally increases the stress already being experienced by the plant.

Vines are generally quite heat-tolerant and can manage 32–35°C reasonably well, when they have sufficient water and can cool themselves. But beyond certain combinations of leaf temperature, VPD and soil-water deficit, they stop photosynthesis, suspend shoot and berry growth, shed or scorch leaves; lose hydraulic conductivity and dehydrate berries.

Lower productivity

In 2026 this has led to the point at which Bordeaux producers are expecting yields of 25-30 hl/ha, rather than 40hl/ha. At best. Economically, at 25 hl/ha, a Bordeaux estate that normally produces 200,000 bottles is currently looking at 130,000.

So, everybody is praying for rain. But water falling out of the sky may not solve their problems. Waxes and organic compounds coating soil particles can become water-repellent during prolonged drying and heat. Water then beads or moves sideways rather than entering the soil evenly. USDA research has found that a second rain may infiltrate better than the first.

Falling through the cracks

Sand and gravel will allow the water to seep in, but may not hold the moisture. Clay soils shrink and crack during drought. At first, these cracks can swallow large quantities of rain rapidly, but the water may bypass much of the upper root zone, travel rapidly down a few preferential pathways, leave large blocks of soil between the cracks relatively dry and/or descend below the depth of active fine roots. As clay wets and swells, the cracks close, allowing subsequent rainfall to run off. So, in a worst case scenario, long drought is followed by a violent thunderstorm, rapid runoff and erosion, and renewed heat.

So the more precise prayers are for gentle initial wetting, further moderate rain and then time for water to redistribute through the soil profile.

Not always a balm

When rain does come, it can create a complex phenomenon called the Birch Effect, in which nutrients are released too quickly for the vine’s damaged roots to capture them efficiently. Among the consequences can be excess canopy vigour and a reduction in berry anthocyanin accumulation in red varieties. All of which creates plenty of work for vineyard managers.

Finally of course, the dry conditions make it easier for fires to grow in intensity and area, which, in turn, leads to risks of smoke taint.

Which is where we came in.

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