Macro, Meso, and Micro Climates: The Importance of Trees in Urban Climates

Guest essay by Dr Tim Ball

I think that I shall never see a poem lovely as a tree, –Joyce Kilmer

Is dishwater dull? Naturalists with microscopes have told me that it teems with quiet life. – G. K. Chesterton.

Climate science is essentially limited to macroclimate, that is global, or at most hemispheric or continental. It’s primarily due to the influence of the Intergovernmental Panel on Climate Change (IPCC) – they can’t do small, or even medium. They rely on models and, as they acknowledge, spatial resolution, “…is generally not high enough to resolve tropical cyclones, and especially to simulate their intensity.” Figure 1 shows a traditional division of climatology studies. IPCC models can’t even encompass Mesoscale, because a single rectangle in their grid can include Plain, Mountains and Basins.

Figure 1: “Area scales of climatic investigation”

Source: Climatology, Oliver and Hidore, P.163.

Part of the Wegman Report that investigated the Hockey Stick fiasco, concluded,

We believe that there has not been a serious investigation to model the underlying process structures nor to model the present instrumented temperature record with sophisticated process models.

Investigation of “underlying process structures”, is virtually impossible because data is inadequate. It is also why even regional models fail. In 2008 Tim Palmer, a leading climate modeler at the European Centre for Medium-Range Weather Forecasts in Reading England, said in the New Scientist.

I don’t want to undermine the IPCC, but the forecasts, especially for regional climate change, are immensely uncertain.

Urban Climates And Vegetation

Difference in temperature between urban and rural areas, known as the Urban Heat Island Effect (UHIE), was first measured by Tony Chandler in London England and described in his 1965 book, The Climate of London. The major reason for the temperature difference is replacement of vegetation with impervious surfaces like asphalt or concrete.

The center of the city, or Central Business District (CBD), is 100 percent impervious surface, while the average suburban property is approximately 40 – 50 percent. (Figure 2).

Figure 2: General suburban lot with impervious surfaces.

Source: Author

Reduced grass and tree cover means quicker runoff and reduced evaporative cooling, among other factors. Treed and grassed areas in cities are measurably cooler than the surrounding urban area. In heat island studies of Winnipeg, Manitoba in the late 1960s we found a one-city block park (Figure 3, marked with green tree symbol) measurably, about 1°C, warmer in winter and cooler in summer. Others had similar results, such as Tim Oke’s measurement of the impact of Stanley Park in Vancouver.

Figure 3 shows the Elm trees lining many streets of older Winnipeg. The City is on the edge of the Tall Grass Prairies. Trees, mostly Quercus macrocarpus (Scrub or Burr Oak), only grow along watercourses. Winnipeg is on the northern limit of these trees and they grow very slowly, so from early days Elm trees were planted along streets, starting around the Legislative buildings. They create a microclimate within the urban local climate.

A study to determine the role of vegetation in CO2 levels in urban areas was useful, but limited. The author’s concluded that, “The amount of CO2 taken up by vegetation in the suburban area was not enough to balance out, or “offset”, the total amount of CO2 released by burning fossil fuels over the course of the year. “Unfortunately, far from it,.. We will still need to find ways to lower our carbon footprint.” What if they found that the vegetation absorbed all the human sourced CO2?

The objective of arguing for increased green space is commendable, however, other, equally important questions need answers. How much do the increased CO2 levels increase urban temperatures and enhance vegetative productivity? How much of the increase is due to increased precipitation? Atkinson demonstrated, as early as 1968, higher precipitation amounts in urban environments. It is due to more condensation nuclei, higher water vapor levels and greater uplift from convection and airflow over the urban dome. How much of increased vegetation is due to a longer growing season?

The role of urban vegetation was a primary concern of Kratzer’s studies in the Ruhr Valley in the 1930s. He was interested in the impact of pollution on trees and the temperature discoveries were coincidental. It was also included extensively in Geiger’s work, Climate Near The Ground, particularly in Chapter 38.

Many are concerned about forests, but urban trees are, relatively, more important. Urban environmentalists constantly complain about forestry, but extent of forests has increased as much as 30 percent in most continents. We are close to a renewable cycle. Replanting forests (silviculture) in practice for some 60 years, means forests are reaching harvestable size, even in slow growth areas. Forestry is close to becoming forest agriculture.

I had the privilege of giving the keynote address at the first conference on urban forests in Canada. Ironically, it was shortly after I’d spoken to a conference at the University of Toronto, which was closing its forestry faculty. Authorities decided forestry was unnecessary in an urban university – what an opportunity they missed. It illustrates how out of touch universities are with the world outside the Ivory Tower. It’s why the phrase, its purely academic, means it is irrelevant to the real world. As someone said, “A professor is a person whose job is to tell students how to solve problems of life which they have tried to avoid by becoming a professor.” (Updated).

Trees also cleanse the air. They transpire oxygen, water vapor and other gases, but also absorb CO2 and other gases. Sometimes this “exhale” is visible, which is why there are so many “blue” mountains around the world, such as the Blue Mountains of Kentucky. Some species survive rigors of urban air and often become symbols of the city. The Plane tree, that Americans call the Sycamore, lined the streets of smog bound London and survived because the bark sheds, thus removing the pollution. Interestingly, this changing color of bark was used to illustrate evolution, because peppered moths came in light and dark. Tree bark darkened by industrial soot, required camouflage change. Dark blended and thrived, light became apparent and vulnerable, so numbers decreased.

The second line of Joyce Kilmer’s famous poem is about the tree’s dependence for sustenance from the ground. “A tree whose hungry mouth is prest against the earth’s sweet flowing breast.” A tree structure includes the trunk that supports the canopy with both supported by the root system. It usually divides into two parts, the taproot that anchors the tree, and the spreading roots that add stability and are the major source of water (Figure 4). When rain falls on the tree, a portion is intercepted and evaporates back to the air; the rest is shed to the outer rim of the canopy. The water is shed to the outside to feed the spreading roots.

Figure 4: Tree root system exposed by erosion.

For many urban trees this is a problem because the impervious surface, too often, goes right up to the trunk. Figure 5 shows a tree in downtown Victoria, British Columbia, but you can see this in most cities. It clearly limits the ability of the tree to get water, but most vegetation put down roots a long way. For example, one species near the Sahara had roots down 100 m. During the Prairie drought of 1988/89, University of Manitoba researchers found wheat roots down 3 m. City trees do the same thing, but it takes a delightful form of revenge. City engineers tell me roots work down into sewer and water pipes causing many problems. It’s only fair that trees get the water engineers denied them in the first place.

Figure 5: Tree on Fort Street, Victoria BC. Notice the water in the gutter that is unavailable to the tree.

Source: Author

Anthony Watt’s examination of US weather stations showed how changes in the immediate area affected results. Still the Global Historical Climate Network, NASA GISS, and all weather agencies assume the station is representative of a much wider area. IPCC and others claim the data is representative of a 1200km radius area. Draw a 1200km radius around your community and decide for yourself. Apart from anything else, Tisdale showed that using a 1200km radius compared to a 250km radius, introduced an artificial warming.

“Using these two extremes, the GISS 1200km radius smoothing contributed more than 20% to the maximum global temperature rise illustrated.”

The Devil is In the Detail

Climate science, as opposed to Climatology, fails because it studies individual components of a vast, complex system. Individual specialists study individual pieces of the system, simplistically depicted in Figure 6. It also fails because it does not examine the different scales and spatial relationships in Figure 1.

Figure 6: A simple systems diagram of climate.

Source: After; Climate Stabilization: For Better or for Worse? William W. Kellogg and Stephen H. Schneider, Science, Volume 186, December 27, 1974

There is a reason why climatology evolved as a major part of Physical Geography. Intellectually it requires a generalist discipline, but specialization dominates today’s dissected, fractionalized, academic world. A more archaic term for geography is chorology. It is defined as “the study of causal relations between geographical phenomena occurring within a particular region.” that seems to summarize what links Figures 1 and 6. Maybe the adage that, we can’t see the forest for the trees, is appropriate?