I just tweeted to all my friends, ’The first frost - 0°C in the greenhouse!’. I was reading from the automated temperature gauge that gives me a warning if the temperatures are low on the rooftop plot. But when I got up there a different scene greeted me. Nowhere could I find evidence of frost. The sweet peas planted late last year are growing well. The Osteospermum is flowering! Nasturtiums are in bud. There isn’t a limp leaf anywhere to be seen.
I’m pretty sure that it was a false alarm. The greenhouse temperature is now 16°C, the sun is shining, there is blue sky. Could it have frosted overnight? Or could I have misread the thermometer? The maximum-minimum thermometer in the greenhouse has gone nowhere near to freezing.
Here, in the centre of London I can only go on my own measurements. Even local weather stations only half a mile away from me record quite different temperatures. This is the urban heat island effect. It can be very localised, influenced by the specific arrangement of air conditioning units, massive concrete constructions, heat absorbing surfaces and the lack of vegetation. Our area is typical of a hot-spot. And what a hot-spot!
For a gardener the effect can be benign. It means I can grow sweet peas all year round. It means I can plant early and harvest late into the autumn. But behind this favourable temperature pattern there lurks risk.
The US Environmental Protection Agency states that urban temperatures can climb between five and ten degrees Fahrenheit higher than the surroundings. I’ve certainly observed temperatures that are 5°C higher than those recorded in the Home Counties, especially on cold nights. Over the course of a sunny day the urban heat island here absorbs heat from the sun, and then at night, acting like a giant storage heater it gives that heat back, keeping night-time temperatures above freezing. This effect does not only happen in urban contexts. In Dorset, the subtropical gardens of Abbotsbury benefit from proximity to the massive shingle embankment of Chesil Beach, a ten mile long pile of stones, that causes a similar effect. (Warm air from the sea adds to the beneficial effect for plants, providing extra humidity as well as warmth.)
Some scientists have posited that in observing the urban heat island effect we can predict the effect of global warming. We can see what a few degrees rise in temperature might bring. Sadly it will not mean that Britain once more basks in sub-tropical seas, as she did during the Carboniferous period, three hundred million years ago. Global warming is far more likely to result in dangerously high temperatures in our cities.
US researchers have discovered that the effects of summer high temperatures in our cities are already leading to more deaths that winter cold, typhoons or hurricanes. (Source: US Environmental Protection Agency) The smog that enveloped Paris last year was a product of temperature inversion, that was precipitated by the urban heat island effect.
During early March 2014 the city of Paris experienced a spate of high pressure, just as we are experiencing now. It brought warm days and cool nights. The high pressure system made for clear skies that facilitated ground cooling at night, while the air above retained the heat released from paved surfaces, heat that was stored during the sunny days. This, combined with a lack of sufficient wind, acted together to trap air pollution beneath a warm cushion of air – an inversion level. As the air was trapped so was the pollution. Sunlight exacerbates the situation providing energy to fix pollutants, increasing them, while at the same time the temperature inversion doesn’t let them dissipate.
By the 16th March the situation had become so grave that Parisian hospitals were full of people suffering from respiratory illness. The city did everything it could to reduce emissions of pollutants. On that day the Guardian reported,
“The city is doing everything it can to discourage the use of automobiles, with tailpipe emissions being the main source of the pollution. Interventions include providing free public transportation for tourists, as well as gratis bicycles and electric cars. Other measures include lowering the speed limits, appealing to trucks carrying heavy loads to refrain from driving, and asking factories to reduce work hours.”
Sitting today in my studio, basking in warm winter sunshine, it seems difficult to believe that such benign weather effect might be leading up to such horrendous conditions.
The solutions are simple. More green roofs, more trees, less hard surfaces. But the most important solution is to reduce the need for air conditioning and comfort cooling in our cities. (Good design can completely obviate this) The waste heat that these units push out is simply that – a waste. And we have to tackle the other end of the cause. I don’t think that petrol or diesel engines should be allowed in our cities. Where I live the potential for electric delivery and low carbon public transport is already well understood. Cycles and electric cars should be the norm, not the exception.
This will mean changes of attitudes. At the moment the gold standard for high quality office means air-conditioned offices. In a temperate climate this is just stupid. The norm for a taxi must never be diesel – the fuel that pumps out particulate matter that lodges in our lungs as violently as if we smoked.
The UK national average of women’s life expectancy is 82.3 years; for men, it is 78.2 years – just over four years shorter. However, in London, the average life expectancy for men is 77.1 years, and for women it is 81.7 years. This lowering of life expectancy is laid at the door of pollution. My garden is doing it’s (miniscule) bit to reduce the urban heat island effect around me. I ‘m living my life trying to reduce my carbon footprint. I can show the way – but it is only if everyone in Britain starts to think like me, and act like me, that we might really change things.