Hotter towns and cities with more variable rainfall are now widely seen as an inevitable consequence of climate change. Indeed, the revised UK 21st Century Climate Change Scenarios 2008 (UKCIP08), produced by the UK Climate Impacts Programme (UKCIP) and due out in November, is expected to show greater temperature rises over the coming decades than previously anticipated.
Many individuals and businesses in the tree and landscape world already take it for granted that greater urban greening, particularly the use of trees, is key to mitigating these trends. But at last month's Trees: The Key to Climate-Proofing our Cities conference, organised by Treework Environmental Practice, speakers pointed out that policy-makers and the public still need persuading.
Indeed, the Trees in Towns II report published earlier this year pointed to a possible overall decline in urban tree cover.
"The issue should be a no-brainer, but we are not achieving that," says broadcaster and government adviser Professor Chris Baines. "We need to be better at communications. But it's hard to depict a bit-by-bit process as a crisis - no one else sees the gradual loss of tree cover in many neighbourhoods. Like marine conservation, it's hard to convey what's going on below the surface."
Conference joint organiser and tree consultant Jeremy Barrell says: "The current balance between trees and other competing interests is not in the best public interest.
"Councils are failing to keep trees on development sites or making effective provision for new planting; highway authorities are not replacing lost trees in our streets; insurers are not accurately valuing trees when assessing claims; and the legal system is not sympathetic to trees."
Dr Roland Ennos and his team at the University of Manchester are attempting to work up a more solid case for an increase in urban tree canopy to take to policy-makers.
"To influence policy, you need numbers," he explains. "For example, we have calculated that there is likely to be a temperature rise of 3-4 degsC in Manchester, and that maximums will be even higher. In addition, we will see drier summers, wetter winters and larger storms."
But he cautions: "There's a lot more work to do to demonstrate these things to the public, to planners, and to members of the city council - some of whom want Manchester to be hotter. And trees are not always so popular with the public either. They are more aware of the dangers than the benefits. Things like cooling and absorbing run-off are harder to show to people."
They are also hard to demonstrate scientifically, he adds. "It is very difficult to carry out properly controlled experiments in cities, comparing tree-covered areas with identical areas without trees. Cities are simply not set up like experimental plots."
Ennos' research has instead used environmental modelling to assess the impact of trees, although he concedes: "Unless one undertakes extensive validation, one is never certain that one's models are producing accurate results."
Funded by UKCIP and the Engineering & Physical Sciences Research Council (EPSRC), the Adaptation Strategies for Climate Change in the Urban Environment (ASCCUE) study of Manchester could be the first time a UK city's green space has been comprehensively researched in this way.
Future validation of the findings using aerial infrared images of the city is planned, Ennos adds.
The team first mapped out the current extent of the city's greenery. "It's actually surprisingly green," he says. "Trees cover 11 per cent of the Greater Manchester area, and 16 per cent in urban areas, though only five per cent in town centres.
"Unfortunately, the trend in many urban areas is currently for people to concrete over front gardens, for councils to remove street trees and for waste ground to be developed - all of which have resulted in a loss of green space in recent years."
Different land use types showed marked differences in temperature, confirming the "urban heat island effect", he says. "Currently, the maximum surface temperature of woodlands is 18.4-12.8 degsC cooler than town centres, which reach 31.2 degsC." These areas are also predicted to experience a bigger temperature rise in future, he adds.
The effect on the predicted temperature of both increasing and decreasing the tree cover by 10 per cent was then calculated (see graph, right).
"A 10 per cent increase is just about doable and can reduce surface temperature by 3-4 degsC, which effectively could climate-proof the city until the 2080s," Ennos says.
He adds that the time frame for increasing cover need not be huge. "What matters is the surface cover, rather than the tree's age, so a lot of smaller trees may be better than one large one. You also get higher transpiration in faster-growing trees like willow."
He also points to green roofs as well as street trees as a means of increasing this percentage. "Some people say they're a fire hazard, but that's ridiculous," he says.
But trees were found to be less useful in other respects. "Run-off is already highest in the city centre, and higher rainfall could lead to run-off of 82 per cent," says Ennos.
"Unfortunately, changing tree cover by 10 per cent either way makes little difference to run-off. Green space isn't enough to climate-proof cities in that sense. (Trees) just don't absorb enough - you get high levels of run-off even in a forest."
Ennos' team also looked at the converse - how climate change is likely to affect the growth of trees and other vegetation. "Reduced summer rainfall will cause droughts of up to five months, which would reduce the capacity of grass to cool the city," he says.
"The solution would be either to store rainfall run-off for irrigation or to plant more trees, which are less drought-susceptible. They also provide shade, reducing the impact of direct insolation on surfaces, and protect people," he suggests.
Winning the public round is likely to require a different approach. Here the Red Rose Forest's I-trees initiative aims to demonstrate to the people of Manchester the effect of green space, especially trees, on temperature. Funding for the project "is being finalised", says Ennos.
Weather stations will be placed in central Manchester, including St Peter's Square. "It will allow people to compare the surface temperatures in the Peace Gardens and by the tram stops, which have no tree cover," he says. "Surface temperatures provide better estimates of human comfort than air temperature and are less prone to error."
RISKS AND BENEFITS
It is a fact of life for tree advocates that the hazards associated with trees receive much greater attention than the benefits they bring.
Temperatures above 21 degsC, for example, are likely to increase the risk of death, says David Ball, a professor of risk management at Middlesex University. "If you wanted to be a mass-murderer, you could chop down a lot of trees."
Yet urban trees are often under threat - ironically under the guise of public safety, he says. "The Health & Safety Executive gets very anxious and involved with dangerous trees, but why not about their removal, which brings problems that are greater in order of magnitude?"
He describes the modern obsession with "'elf 'n' safety" as "out of control". This ends up having harmful consequences, Ball says.
"It can pit safety from injury against health, so that we end up with safety versus health."
Other areas of public life suffer similar handicaps, he says - pointing to football tournaments being cancelled over "safety fears", which overlook the health benefits of sport.
Ball blames a combination of an increasingly litigious culture and the political popularity of safety-related legislation.
"The safety 'meme' has spread," he says. "Thirty years ago, you forgot about it, now it's the first thing you think about. But if we don't take account of the benefits (of trees), we will make a serious policy mistake."
Trees are capable of all the following, which he says policy-makers need to take into account:
- mitigating the effects of increased temperatures;
- providing shade;
- intercepting rainfall;
- removing respirable particles from the air;
- serving as important components of the eco-system;
- contributing to psychological and physical well-being;
- acting as "carbon sinks" - at least temporarily.