Analysis of air bubbles trapped in polar ice have allowed us to monitor the change in atmospheric carbon dioxide levels over time. Between 150,000-1,000 years ago, analysis indicates that carbon dioxide levels in the atmosphere fluctuated between 200-300 µmol per mol l (this SI unit has replaced parts per million). 1000-150 years ago, atmospheric carbon dioxide levels were 280 µmol per mol. 150-10 years ago, atmospheric carbon dioxide levels were 352 µmol per mol. This increase has been exponential and is accelerating.

In the last 50 years, the atmospheric concentration of carbon dioxide has increased by 30% over pre-industrial levels. The main reasons for this carbon dioxide enrichment have been the burning of fossil fuels and the clearing of vegetation from land particularly tropical forests (Stuiver, 1978).

Carbon dioxide is termed a greenhouse gas. This means that it absorbs long wave radiation more efficiently than short wave radiation. Short wavelengths predominate in sunlight and penetrate the atmosphere warming the Earth. The Earth then returns longwave radiation. This longwave energy is absorbed by greenhouse gases such as carbon dixoide which, in turn, radiate part of the energy (in long wavelengths) back to the Earth so warming it. Experts now agree that global warming will happen and, in fact, it is already occurring. Fifteen of the worlds hottest years, since recorded history have all occurred in the 1990's. This is resulting in well publicised phenomena such as:

• Changes in climate and rainfall pattern.
• Changes in ecosystem composition.
• More extreme weather with, in the UK, wetter winters and hotter, drier summers.
• Increasing severity of 'urban heat islands'.

The projected elevation in mean temperature initially appear small. For North West England mean temperature increases by 2050 of between 0.8°C and 2.0°C are forecast. However, the social, environmental and ecolgical impact of these increases are likely to be significant, complex and worryingly unpredictable.

Sustainable development is the pragmatic application of the philosophy. It has to be accepted that protection of the environment must go hand in hand with allowing both society and business to support itself and develop. If protection of the environment ignores society and business, then society and business will ultimately exclude environmental concerns.

An integral part of the drive for good sustainable practice, particularly in the commercial sector, is accountability for the production of the major greenhouse gas responsible for global warming- carbon dioxide. Good sustainable practice in itself can only reduce the production of carbon dioxide. The only universal process that removes carbon dioxide from the atmosphere is photosynthesis. Photosynthesis is the chemical reaction by which plants use the sun's energy to combine water and carbon dioxide to make sugars for growth. Thus the only process that can claim to remove carbon dioxide is the utilisation of plants. For this reason roof planting on a large scale and collectively driven at the legislative level, could play a crucial role in the sustainable development practices of the future.

• Improved rainwater management
• Improved building thermal performance
• Reduction in sound transmission
• Improvement in air quality
• Green roofs and the urban heat island effect
• Provision of habitat for native flora and fauna

A survey of the causes of poor river water quality in Scotland in 1995 (State of the environment report, Scottish Environment Protection Agency, 1996) found that 20% of the poor quality waters resulted from run off from urban areas. The environment agency has begun to address these issues by identifying alternative approaches which have been collectively referred to as Sustainable Urban Drainage Systems (SUDS) (Sustainable Urban Drainage Systems: an introduction).

The ideal solution would be to control water run off at source. However, it is difficult to install containment measures within the urban area where space is at such a premium. Roof planting offers what may be the only real solution. Green roofs reduce rainwater run off by a number of processes:

• Retention of the rainwater in substrate and drainage layers.
• Uptake and release of water from the substrate by the plant layer. Water is transported from the root to the leaf and lost through the leaf surface - a process known as transpiration.
• Solar driven evaporation of water from the substrate.
• Wind disruption of the still, humid air layer (known as the boundary layer) over the plants and substrate increasing the rate by which water is lost to the atmosphere through evaporation and transpiration.

In addition, vegetation and substrate can absorb a range of pollutants including nitrogen, phosphorus and heavy metals such as cadmium, copper, lead and zinc.

In many cities around the world it has been recognised that the most significant ecological advantage of roof planting is storm water management. It is hard to come up with average figures of performance, however, roughly 50-60% of rainfall can be expected to be retained by an extensive green roof. Trials carried out at Trent University in Nottingham found that roofs fitted with a vegetation layer growing on a lightweight aggregate substrate layer considerably reduced the volume of rainwater discharged. Expressed as a percentage of the unplanted control roof, the planted roof retained 100% of a 3 mm rainfall, 80% of a 3-23 mm rainfall and for one period where 41mm of rain fell over 47 hrs, 73% of the rain was retained. Obviously, the planted roof is not designed to act as a sponge and eventually the roof will become saturated and discharge water.

Obtaining definitive figures for rainfall retention and discharge rates is difficult because trial observations are affected by a number of factors:

• The water status of the system prior to the rainfall. How much water is in the system at the start of any rainfall event will have a direct effect on the quantity of water retained during that event.
• How well established the vegetation layer is. The root systems of plants can retain very significant amounts of water. Shoot systems can take up and transpire (lose) very large quantities of water. A newly installed planted roof will not retain as much water as an established roof.
• The intensity of the rainfall. The ability of the planted roof system to retain water will be different for a long period of gentle rain compared to a short period of torrential rain.
• The season. Evaporation of water from the substrate and transpirational loss from the vegetation layer will be much greater in the summer due to the higher temperature and increased plant growth.
• The pitch of the roof. The greater the roof pitch, the less water is retained.
• The species of plants that make up the vegetation layer. The capacity to take up and transpire water will vary from species to species and also with season.

Trials we have carried out on our own systems indicate that between 40-60% of rainfall is retained and the run off time of the remainder is greatly extended. We are continuing to research into the effects of the above factors and into methods of optimising this aspect of the system taking into account other factors such as weight and plant performance.

Once again, Germany has taken the lead by recognising at legislative level the collective benefits of roof planting with regard to rainwater management. In several cities a tax on commercial buildings related to the amount of sealed ground space they occupy is now levied. In Berlin, this tax amounts to 3-4 DM per m² per annum. A reduction of 50% of this tax rate is applicable for buildings that have planted roofs. Similar taxation and incentive schemes operate in Bonn, Munich and Stuttgart. For these reasons the roof space covered by greenery in German cities has increased at an astonishing rate. In 1995, 10,000,000m² of roof space had been greened. By 1999 this figure had risen to 84,000,000m².

Although in the UK we lack any legislative drivers for roof planting, the pressures on development to accommodate it are increasing, if indirectly. The following is a section from an environment agency directive regarding a large commercial project we have been involved with:

'The discharge from the developed site should mimic that from the existing greenfield site as far as is practical. Thus in a 1 year event the discharge should be the same as for the greenfield site, and in a 10 year event, and similarly in a 100 year event. So across the range of return periods the natural greenfield runoff is simulated'.

The architects are concerned about the expense of ground based containment areas that will satisfy the directive. As discussed here, keeping the water on the roof and returning it to the atmosphere is a cost effective solution and an attractive one in the light of the directive.

With any collective benefit such as better rainwater management, a government led initiative is imperative. As an indication of the scale of importance of roof greening with regard to this issue, it is of interest to note that the area of low pitched roofs suitable for extensive type planting with little or no structural modification in UK city centres alone has been estimated at 200,000,000m² (The Successful Green Roof. Roofing, cladding and insulation, Hooker,1994). Put another way this equates approximately to the entire land area of the city of Manchester!

A. Summer
The roof can suffer from huge thermal fluctuations on its upper surface throughout the day and through the year. In extreme cases these can range over 100 °C. (Papadopoulos and Axarli, 1992). Planting the roof surface dramatically reduces the amount of solar radiation absorbed by the roof's bare surface. The high daily thermal swings are neutralised and the annual fluctuations are decreased to between 20 and 25°C.(Eumorfopoulou and Aravantinos, 1998). Planting the roof reduces heat build up by the following mechanisms:

• Photosynthesis. This is the process by which plants use the sun's energy to turn water and carbon dioxide into sugars. Thus, some of the solar radiation is 'diverted' to drive the plant's growth rather than heat up the roof.
• Evaporative water loss from the plant tissues. Converting water from liquid into gas requires energy. Plants use this process to prevent over heating. The sun's energy is used to evaporate water rather than cause heat build up within the plants tissues. Evaporative water loss from plant tissue is termed evapo-transpiration. The solar radiation is therefore 'diverted' into evaporating water from the plant tissues rather than heating up the roof surface.
• Evaporative water loss from the soil or substrate. Much as for the plant tissue, solar radiation is 'diverted' into evaporating water from the soil or substrate rather than heating up the roof surface.

Studies carried out at Trent University under British climatic conditions, indicate that planted roofs can have markedly lower temperatures throughout the roof layers compared to the unplanted roof. With a mean daily air temperature of 18.4°C, the temperature below the membrane surface of an unplanted membrane roof typically reaches 32°C compared to 17.1°C for a roof with an extensive type green roof cover. These results indicate that bare roof surfaces can act as strong heat sinks and reach temperatures far above the ambient even under the quite mild climatic conditions of the UK. This makes it harder to control the ambient temperature within the building. Planting the roof surface can dramatically reduce the heat sink effect of the roof thus reducing the amount of energy needed to control ambient temperatures inside the building- a conclusion also reached by the administrative body of the City of Chicago! (see 'urban heat island effects'). As well as being economically advantageous as less energy would be spent on air conditioning, the reduced energy requirements of the building would mean less production of carbon dioxide (the main greenhouse gas contributing to global warming).

Looking to the future, global warming will mean that we will all have to cope with the elevated temperatures of our environment. Present government directives on building performance mean that many of us will be doing this in some of the best insulated buildings ever seen in this country. The accelerated need for energy expenditure (and therefore carbon dioxide production) to provide air conditioning for these, and all, buildings will increase and this will further fuel climatic change. Lightweight planted roof systems can only increase in importance as THE sustainable solution for keeping our buildings workable and liveable in the years ahead for two reasons:

• Reduced carbon dioxide production by the building less energy utilized by the building means less carbon dioxide produced.
• Removal of carbon dioxide by the roof plants The fixation of carbon dioxide by the roof plants to form sugars and the release of oxygen are two vitally important environmental benefits of roof planting.

B. Winter
Planted roofs do reduce the buildings heat loss through the roof during the winter months. This is achieved by a number of processes:

• The insulating effects of the added mass and individual properties of the system components.
• The biological activity in the root zone of the planted system which generates heat.
• The creation of a still layer of air (boundary layer) over the plants that reduces thermal loss.

It should be noted that the insulation effiency of a planted roof depends on the amount of water held in the substrate and plant layers. The water has a negative effect on thermal conductance. Thus, a saturated plant system will insulate less than a dry system. Should the water freeze in the substrate layer this will further increase thermal conductance and therefore reduce any insulatory effect. For this reason, it is not possible to provide absolute figures as to improvements in thermal performance.

In spite of this, benefits of planted roofs as insulation against heat loss have been recorded. Work at Trent University indicated that on a cold day in the UK with a mean temperature of 0°C, the temperature under the membrane of an unplanted roof system fell to -0.2°C. By contrast, the temperature directly under the membrane of a planted roof had a mean value of +4.7°C.

It can be concluded that roof planting will result in reduced thermal gradients between the interior and the exterior of the building through out the year. Reducing this thermal gradient means that roof planting lessens the driving force for heat loss from the building's interior to the exterior when external temperatures are low and for heat gain from the exterior to the interior when external temperatures are high. Roof planting means that energy requirements by heating or air conditioning systems to maintain acceptable ambient temperatures within the building, will be reduced.

• Plants remove atmospheric carbon dioxide.
• Plants release oxygen.
• Plants and substrate release water vapour so humidifying the air.
• Airborne particulate pollutants are deposited in the substrate, on the leaf surfaces of the plant layer, and onto the moist internal surfaces of the leaf.
• Air borne heavy metals are absorbed onto plant and substrate surfaces.
• A range of organic volatiles including formaldehyde, xylene, toluene and benzene are removed from the atmosphere.
• Large scale roof planting will reduce the 'urban heat island effect' and improve the flow of cool fresh air into the city.

Some of the most comprehensive research carried out on the effects of urban vegetation on particulate pollutants was with regard to parkland trees in cities. Such plants were found to filter out up to 85% of suspended particles (Lohmann, 1990). Similar findings have been cited for climbing plants in urban settings (Doernach, 1979). The filtering is by deposition of the pollutants on the leaf outer surface and by deposition on the wet inner cellular surfaces of the leaf.

NASA has been using remote sensing thermal imaging to examine urban heat island effects for a number of US cities. Project ATLANTA focused on that one US city. In natural landscapes, the plant canopy biomass greatly lowers air temperatures whereas the artificial surfaces common in urban landscapes greatly raises them. The NASA report concluded that correctly sited 'urban forests' are a vital component of keeping cities cool.

NASA figures show intense thermal energy responses from buildings, rooftops, pavements and other urban surfaces. On a typical Atlanta day with maximum air temperature of 25°C(77°F) the following temperatures were recorded:

• Tree shaded grass, 28°C (82.4°F)
• Tree canopy, 21°C (69.8°F)
• Asphalt in full sun, 50°C (122°F)
• Membrane roof surface 52°C (125.6°F)

This data indicates that local micro climates can be improved by the presence of green space.

In downtown Atlanta, the air temperature is often 10°F warmer than the surrounding outlying areas. NASA concluded that concrete, tarmac and bare roof space soak up virtually all the radiation that falls on them and reradiate it as heat. This elevated temperature has profound effects on air quality (for example, it was suggested that it could double the amount of ozone produced). Similar studies have been carried out for Salt Lake City and New York. In the Salt Lake City study roof surface temperatures of 71°C were recorded.

There was evidence that the urban heat island effect seen for Atlanta was even affecting the weather patterns of surrounding districts.

Information from thermal studies, carried out at Trent University in the UK, found that on a typical day where ambient temperature was 18.4°C, a bare membrane roof had a surface temperature of 32°C. An identical roof covered with a thin layer plant system had a surface temperature of approximately 15°C.

One has to conclude that conditions in built up areas could be improved by the use of more plant ground cover. A similar conclusion was reached by the scientists of NASA. The question is, how can a significant enough area of ground space be planted within our intensely developed cities? As with the search for sustainable methods of rainwater management, the solution may be to look to the roofs. Data indicates that the strong heat sink effects seen on roofs in the UK can be greatly reduced by planting. With a collective approach to the issue, planting existing roof space would have a very significant and positive effect on air quality and temperatures within our cities.

The City of Chicago seems to be convinced of the collective benefits of roof planting. As a direct result of problems with elevated city temperatures, the administration has committed over $1,000,000 to the quantification of the benefits of roof planting. The City of Chicago are now issuing 'green roof grants' under its 1999 Urban Heat Island Reduction Initiative.

As well as improving air quality, an energy study conducted by that city estimated that, with the greening of all city roof tops, energy to the value of $100,000,000 could be saved each year because of the reduced demand for air conditioning. It was estimated that the peak demand for energy would be reduced by a colossal 720mW. This reduced demand for energy would also mean a massive reduction of carbon dioxide production- especially significant for the world's greatest producer of this 'global warming' gas.

• They are undisturbed.
• They are free of predators - the main culprit being the domestic cat.
• They can be constructed to cater for selected species. This could include the type of vegetation planted on the roof and the plant and animal species being encouraged to colonise it. For example, the provision of bare areas for nesting and plenty of small areas of short vegetation can encourage the Black Redstart. This bird species is considered endangered in this country but has been found colonising green roof space in South East England.

The interaction of birds, beetles and spiders with European green roofs has been studied by Stephan Brenneisen at Basel University. The major findings were that:

• Appropriately constructed green roofs attract birds. These birds use the roof to search for food (insects and seeds), preening and cleaning, searching for nesting material, nesting and roosting. Wheatears, skylarks, crested skylarks, lapwings, common terns and mallard have been recorded as nesting on European green roofs. The pied wagtail requires open areas of ground with no vegetation. Goldfinchs require a varied plant layer to supply the range of seeds that it prefers. An integrated policy on green roof construction would allow linked areas of habitat to be created. This is vital to encourage dwindling bird species back into the cities.
• The capacity of certain material to retain water is vital as this encourages the beetle population BUT will also affect the plant layer. In this European study 172 different species of beetle were found on green roofs including many rare examples. The older roofs had the greatest species range. Small hills in the substrate allowed moisture retention and this encouraged the beetle population. Green roof relief, thickness and substrate had effects on the beetle population.
• In this study 60 different species of spider were found on green roofs.
• Natural soil incorporation into the substrate is good for the encouragement of biodiversity.
• Areas that retain water during dry periods are helpful. In Luzern, orchids have been found to have colonised certain roofs.
• The incorporation of larger stones and pieces of timber is beneficial as this provides localised humid sites for beetles and spiders and perching points for birds.

The encouragement of biodiversity by way of roof top planting is becoming more high profile, particularly in the highly urbanised South East of England. It has been a worrying trend that brown field sites around and within urbanised areas are viewed as easy prey for development. Brown field sites do provide valuable habitat for plant insect and animal species and the replacement of this habitat on the roof may be a solution.

A thorough understanding of how the 'habitat' green roof is to integrate with local flora and fauna is critical. At BHC, we are conducting trials with combinations of substrate formulation and plant species with particular regard to 'habitat' roofs being a subtle but distinct form of green roofing in the UK.

Until quite recently, the importance of green roofs in such schemes has been overlooked by policy makers but increasingly, their role in sustainable urban drainage schemes, promotion of biodiversity and building performance is being recognised at many levels.The Mayor of London, Ken Livingston and his chief advisor on architecture and urbanism, Richard Rogers, issued a statement in 2004 promoting the use of green roofs in London. The Greater London Authority (GLA) campaigns for green roofs through mayoral policies and strategies (the London Plan, the Mayor's Biodiversity and Energy Strategy, and planning guidance on sustainable design and construction) and is encouraging the Government to establish practical ways of supporting living roofs across the UK. The GLA is also one of the sponsors of a Biodiversity and buildings project run by CIRIA (the Construction Industry Research and Information Association).The objective is to provide guidance on the technical, structural and planning issues that should be considered in integrating ecological and sustainable drainage features (primarily green roofs) into building design.This guidance will be of interest to developers, RSLs, designers, consultants and planners and is scheduled to be made available in the summer of 2006.

CABE (the Commission on Architecture and the Built Environment) is a non-departmental public body set up by the government in 1999. Through public campaigns and supporting professionals it 'encourages the development of well-designed homes, streets, parks, offices, schools, hospitals and other public buildings'. In a new report Creating Successful Neighbourhoods, launched on 1 February 2005, the CABE highlights how the adoption of design innovation is leading to early success in the Government's nine Pathfinder areas. The Pathfinders were first announced in February 2003 as part of the Sustainable Communities Plan. The projects form the first phase in a 10- to 15-year programme of regeneration and housing improvement. The new report from CABE reflects on the progress of the Pathfinder programme. It highlights redevelopments such as that in St Mary's in Oldham which puts sustainable urban design at the heart of the scheme, with the aim to achieve an EcoHomes excellent rating. Features such as green roofs, solar water heating and wind power are included in the proposals. Environmental Design Consultants are supporting the work of the architects.

In general, architects are increasingly aware of the importance of environmentally aware design and we have noticed a growing interest in green roofs.