Kapur trees Dryobalanops aromatica in Malaysia avoid overlapping crowns as they grow, which creates a jigsaw-puzzle pattern when viewed from below. When it comes to fighting global warming, trees have emerged as one of the most popular weapons. With nations making little progress controlling their carbon emissions, many governments and advocates have advanced plans to plant vast numbers of trees to absorb carbon dioxide from the atmosphere in an attempt to slow climate change. But emerging research suggests that trees might not always help as much as some hope. China aims to plant trees over an area up to four times the size of the United Kingdom.
California is allowing forest owners to sell credits to CO 2 -emitting companies, and other US states are considering similar programmes, which could motivate projects that establish new forests and protect existing ones. The European Union is moving towards allowing countries to include forest planting in their plans to fight climate change; some nations in the bloc have also pledged billions of dollars to tropical forest programmes. Many scientists applaud the push for expanding forests, but some urge caution.
They argue that forests have many more-complex and uncertain climate impacts than policymakers, environmentalists and even some scientists acknowledge. Although trees cool the globe by taking up carbon through photosynthesis, they also emit a complex potpourri of chemicals, some of which warm the planet. The dark leaves of trees can also raise temperatures by absorbing sunlight. Several analyses in the past few years suggest that these warming effects from forests could partially or fully offset their cooling ability.
Such concerns have prompted vigorous debate among scientists about how forests in different regions have warming or cooling effects.
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And no researchers are suggesting cutting down existing forests or curtailing efforts to combat deforestation. But as governments, corporations and non-profit organizations advance ever-more ambitious programmes to slow climate change, some scientists warn against relying on forests as a solution to global warming until a better understanding emerges. Researchers are involved in major campaigns to collect data using aeroplanes, satellites and towers in forests to sample the full suite of chemicals that trees emit, which can affect both climate and air pollution.
At the same time, some researchers worry about publishing results challenging the idea that forests cool the planet. One scientist even received death threats after writing a commentary that argued against planting trees to prevent climate change. The questions are multiplying as more scientists enter the debate.
At the same time, increasingly dire warnings about climate change — and the potential for huge amounts of money to go towards planting forests — have made working out how trees affect climate a matter of urgency. If tree-planting programmes work as advertised, they could buy precious time for the world to reduce its reliance on fossil fuels and replace them with cleaner sources of energy.
Although the analysis relies on big assumptions, such as the availability of funding mechanisms and political will, its authors say that forests can be an important stopgap while the world tackles the main source of carbon emissions: the burning of fossil fuels. The first inkling that plants suck CO 2 from the air dates back to the s, when Swiss pastor Jean Senebier grew plants under different experimental conditions. He suggested that plants decompose CO 2 from the air and incorporate the carbon, an idea corroborated by subsequent discoveries about the mechanisms of photosynthesis.
The rationale is that trees can lock up carbon in their wood and roots for decades or even centuries. The climate treaty known as the Kyoto Protocol allowed rich countries to count carbon storage in forests towards their targets for limiting greenhouse-gas emissions. Later negotiations laid out a framework for enabling wealthy countries to pay poorer tropical countries to reduce emissions from deforestation and to increase carbon in forests.
Source: X. Song et al. Nature , — Such schemes required firm data on how much carbon is locked up in forests. In the past few decades, scientists have worked to create national estimates of carbon loss and gain from vegetation by studying field plots and by combing through satellite data. Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground.
This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-coloured ground or snow that would otherwise reflect sunlight. Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.
More-recent studies have branched out to include other ways in which forests can influence climate. As trees live, grow and die, scientists have learnt, they are in constant conversation with the air, swapping carbon, water, light and a bewildering array of chemicals that can interact with the climate. Atmospheric chemist Nadine Unger, then at Yale University in New Haven, Connecticut, conducted one of the first global studies examining one part of this exchange: the influence of volatile organic compounds, or VOCs, emitted by trees.
These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas.
Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight. Unger ran an Earth-system model that estimated the effects of chemical emissions from forests. Her results suggest that the conversion of forests to farmland throughout the industrial era might have had little overall impact on climate 3.
As a corollary, Unger suggested that reforestation would also have uncertain climate effects. Trees in tropical and temperate zones emit huge quantities of isoprene that is not accounted for in most forestry schemes. She acknowledged that her study was a first step, and called for increased monitoring of forest chemicals and their atmospheric interactions.
Linking Climate Change to Land Surface Change
The article, and especially the headline which Unger did not write , triggered a tsunami of complaints from researchers, who disputed the science and said the piece threatened to undermine years of research and advocacy. At metres high, the Zotino Tall Tower Observatory measures gases and aerosols above taiga forest in central Siberia. A similar tall tower in the Amazon makes measurements above the tropical rainforest.
Unger says she received death threats, and that some colleagues stopped speaking to her. Some scientists, however, agreed that it was important to look at the impacts of forest VOCs. A team led by Dominick Spracklen and Catherine Scott, atmospheric chemists at the University of Leeds, UK, ran a model that included how aerosols from forests can seed clouds, which reflect sunlight. They concluded that the net effect of VOCs from forests is to cool the global climate 4. Unger, who is now at the University of Exeter, UK, and Spracklen are discussing using a common experimental design to try to resolve their differences.
They and other researchers say that such studies are hamstrung by sparse data sets on forest emissions. The latest findings are piling on even more complexity. Ecologist Sunitha Pangala at Lancaster University, UK, spent much of and in the Amazon rainforest, where she placed gas-measuring chambers around the trunks of more than 2, trees. Researchers had previously assumed that methane leaked into the air directly from the soil, where it is produced by microbes. The new work suggests that trees could be another conduit for that microbial methane, potentially explaining why more methane has been detected above tropical wetlands than has been measured emanating from soil alone.
Similarly, climate change is likely to increase exposure to either natural or economic hazards, all the more so because in many mountain areas, poverty levels are higher than in lowland areas and food insufficiency is more widespread Ives and Messerli, ; Kohler et al. In fact, predicting future climate trends relies on both a large network of meteorological stations and modelling outputs from satellite-derived data Beniston, ; Immerzeel et al. Yet, trends can currently be generated only on a decadal time scale.
It aims to provide some answers to the following questions:. What are the different methods to assess changes in climate? Which are the best, most accessible and integrative ones? What are the best field indicators of climate change? How can the impacts caused by climate change, in the strict sense, be differentiated from those linked to land use change?
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Do mountain populations perceive any evidence of climate change temperature, precipitation amount, seasonality and extreme events at a local scale? Do they feel threatened? How do policymakers reconcile socio-economic assets ski resorts, water supply, etc. It is mainly a survey of the observed and projected changes in climate patterns, cryosphere dynamics, and derived and expected natural hazards floods, debris flows, landslides, and rockfalls. These authors review existing databases on climate change and natural hazards at different levels i. Some significant socio-economic impacts are also mentioned, in relation to potential extreme hazards, in order to urge stakeholders and policymakers to anticipate and adapt to these new, foreseeable situations.
At high elevations, one good indicator of climate change is permafrost, a major element of the mountain cryosphere whose existence is generally best demonstrated by hanging glaciers or rock-glaciers. Any change in permafrost conditions temperature or extent may generate new risks for the surrounding mountain population, infrastructures and territories Haeberli and Beniston, ; Huggel et al.
They summarise research that has been carried out for the last ten years within the framework of PermaFRANCE, a network that was set up to monitor the long-term evolution of permafrost. Temperature measurements include both surface sensor and subsurface borehole methods. The preliminary results obtained for the last 5 five years show a clear tendency of increasing temperatures, which is consistent with data collected from other Alpine boreholes. In their conclusion, the authors emphasise the conditions of stability of alpine slopes, which are clearly modified by the warming of permafrost and related changes in the ice or water contents of soils.
Therefore, understanding and predicting the consequences in terms of risks should take into account the very high variability of the local conditions in order to meet societal expectations, specifically those of natural hazard managers. For each one, they consider a series of factors characterising the geomorphic hazards either passive or active that may cause slope instability.
They also include the potential level of damage intensity, together with an index of value both financial and operating in order to assess the degree of vulnerability better. Finally, they build an index of destabilisation risk to identify and rank infrastructures at risk. Mountains provide water resources for domestic, agricultural, industrial and tourism purposes, and any change in this resource either surface or groundwater may affect water availability and hence any economic activity based on this resource.
This is all the more true as mountain areas now host larger population densities. Four contributions illustrate the varying water uses in different socio-economic contexts and continents. In the Anti-Atlas mountains of Morocco, Aziz and Sadok show how saffron production, a major pillar of the local economy, is a fairly water-demanding crop and as such is directly threatened by climate variability.
More specifically, it appears that the general temperature rise of recent decades has shortened the cold season, and the decrease in snow volume in the mountains has led to a water deficit, and hence a reduction in the economic profit generated by this emblematic cash crop. In order to assess better the local perceptions of climate change, the authors surveyed 60 farmers using questionnaires , and a few stakeholders semi-structured interviews.
In this arid region mm mean annual precipitation studied by Delbart et al. There, the growing population and increasing water demand make access to the water resource a priority, which requires some anticipation. The authors analyse the link between the seasonal and inter-annual variations in river discharges measured upstream of the first dams built on the four rivers feeding the irrigated plots.
In order to forecast the average river discharge during the spring—summer months, they use a remote sensing methodology based on MODI10A2 images period. Despite the period of analysis being too short to conclude there is a significant regime change, the authors show that large differences in discharge are related to the total surface area of the snow cover among watersheds, with a direct link to watershed dimensions.
Impact of climate change on mountain environment dynamics
They also show that the area of the snow bed extent observed at the beginning of the snowmelt period directly influences the total discharge in rivers. In a very original approach, i. From extensive interviews carried in four fieldwork sites representative of Nepalese milieus, they find contrasting situations and changes in practices with no obvious connection to the climate.
Nevertheless, their information collected about snow, a parameter that has been measured incorrectly and underestimated in simulations, shows that populations are more affected by fluctuations in rainfall patterns than by the melting of glaciers and the snow cover. The authors conclude that the population groups most likely to be affected by climatic variations are those living in high mountains and low mountains where the long dry season is quite often problematic , compared to those living in the middle mountains and the foothills where their pluri-activity agriculture, portage and services limits the risk that might be caused by irregular, insufficient rainfall.
More generally, the recent demographic growth, infrastructure development and diversified incomes have appeared as alternative explanations of the changes observed other than the climate. Snowmaking is considered a logical way to extend ski resorts spatially, to compensate for the seasonal deficit in snow and mitigate the effects of climate change. This guarantee of snow represents extra investments that may not be financially viable for all ski resorts in the future. They set up a socio-economic on-line survey for professionals, including ski patrol managers and technicians working in ski resorts, in order to classify the ski resorts according to their size, elevation and equipment number, age and size of ski lifts.
They establish a ratio of equipped surface area that turns out to be larger for large resorts than for small and medium-sized resorts. Therefore, the scarcity of water availability and weather conditions favourable for artificial snowmaking is likely to become an increasingly common situation, representing an increase in costs for ski resorts that only the richest largest and highest will be able to bear. In mountainous areas, two parameters are particularly significant. Firstly, and in addition to glaciers and their accelerated retreat, ground-ice status appears a good indicator of temperature rise and hence a possible cause of slope instability when ground-ice progressively melts, putting tourism infrastructures at risk.
Secondly, the water resource has become the main concern for populations living in the mountains or their foothills. The melting glaciers, and more importantly the decrease in snow volume and duration, and its temporal and spatial variability, will affect the discharges of springs and rivers, and hence the availability of water for mountain people.
In addition, change and variability in rainfall, two parameters not well constrained by global and regional climate models, are certainly key factors that must be better defined at catchment and local scales. Finally, even if some impacts, such as hydro-geomorphic hazards and biodiversity evolution, have not been expressly documented in this volume, we hope that through the different examples developed, some methodological approaches remote sensing, destabilisation risk index, interviews on perceptions might be useful to define the appropriate policies required in order to anticipate, adapt to, and manage the potential impacts of climate change in mountains.
B eniston M. Implications for the Hydrological Cycle and for Water Management. Climatic Change , — In: Huber U. Springer, Dordrecht, pp