Prof. Dr. Andrzej Kędziora, Prof. Dr. Zbigniew Kundzewicz
Research Center for Agricultural and Forest Environment
PolishAcademy of Sciences
e-mail:
Impact of climate and land use changes on natural resources in agricultural landscape.
1. Introduction
Global climate conditions are created and determined mainly by three sets of factors: physical processes and properties of atmosphere, chemical processes and composition of atmosphere and earth surface properties and processes. All these three sets of factors were influenced by human activity, especialy during the last century (Fig. 1). Three processes: energy flow, matter cycling and global atmospheric circulation are the main processes responsible for functioning of the climate system in different scales. The chemical and physical properties (concentration of greenhouse gases and cloudiness) of atmosphere determine the flux of solar energyincoming in the planetary systemof the Earth as well as the sum of energy remaining in the system (greenhouse effect). But, the interaction between atmosphere and the earth surface influences the effect of these three processes.Thermal conditions of the earth surface and lower atmosphere depend mainly on partitioning of solar flux into latent heat (evapotranspiration) and sensible heat (heating soil and atmosphere). In turn, this partitioning depends on earth surface character, mainly richness of vegetation and water bodies. The more intensive evaporating surface the less energy remains for air heating (Tab.1). Bare soil uses 5 times more energy than forest or water body for air heating(Ryszkowski, Kędziora 1987). So, the change of chemical composition of the atmosphere and de-vegetation as well as decreasing water surface caused by human activity brings about the possibility of occurrence of vital changes of climate during last decades. One of the most important transformations caused by human activity was transformation of the stable ecosystems like forests, pastures, water bodies into unstable ones like arable land or urban areas (Fig.2). Such land use changes impact unfavorably on the water balance structure; diminishing evapotranspiration and enhancing run-off (Tab. 2). During a dry year about 20% of precipitation is removed out of the landscape, while meadow and forest keep all the available water. During a wet year crop fields lose as much as 40% of precipitation, while forest only 20%. Thus, forest and meadow are the landscape elements which conserve the water, while crop fields lose water unproductively.At present, there is an ongoing reforestation in Europe, but deforestation prevails in many countries of the Third World.Today, 40% of the Earth’s land surface is managed for cropland and pasture and natural forests cover another 30%. In developing countries, nearly 70% of people live in rural areas where agriculture is the largest supporter of livelihoods (Easterling et al., 2007). This illustrates the importance of agricultural land for the socio-economy and the environment. Agriculture has to feed increasing human population in the decades to come. Yet, since now many people suffer hunger or are undernourished, the Millennium Development Goals to reduce, globally, the number of starving people by half until 2015, will be difficult to achieve. In Poland one can observe increase of grain crop area (Tab.3).Agricultural land makes presently 60% of total territoryof Poland, while cereals grow at 32% of whole area of country. Therefore, cereal fields constitute not only the dominant element of countryside, but also influence distribution of many organisms and affect their prospects for survival and migration.
The influence of the plant cover structure on the sensible heat flux will be illustrated by the results obtained in studies of the heat balance of sugar beet field, located in the vicinity of a field covered by the stubble left after wheat harvest.The active surface of intensively transpiring sugar beet field use much more solar energy for water evapotranspiration than stubble field does. This leads to large differences in surface temperatures of these ecosystems. The difference between surface temperature of stubble and sugar beet fields was up to 6.4 oCduring a sunny day (Tab. 4), while the difference of air temperatures over these fields on the level of 2 m above ground was only 0.13 oC. During a cloudy day the differences were much smaller, reaching only 1.1 oC on the active surface and disappearing on the level of 2 m above ground. The large vertical gradient of the air temperature near the surface strata indicates that on a sunny day much of sensible heat is transmitted from the earth surface to the atmosphere, enhancing the air turbulence. This process intensifies the exchange of mass in the boundary layer e.g. evapotranspiration. Such situation is characteristic for the anticyclonal circulation. In the studied landscape in summer such circulation cases occur during about 40 % of time.
Vertical gradient of air temperature on a sunny day is nearly 9 times higher over a stubble field than over the sugar beet field. On a cloudy day the vertical gradient over stubble field is negative, however 14 times smaller then on a sunny day. In the same time the vertical gradient over sugar beet field is positive. This is, of course, the result of plant transpiration, using more energy than the amount available from the sun. The transpiring sugar beet plants gain the lacking energy from the air causing temperature inversion. Thus bare soils or man-dried surfaces are the areas where convection is generated, which influence the energy and mass exchange at the local as well as a regional scale. Vertical gradient of wind speed was higher over sugar beet field than over stubble field because of greater roughness of sugar beet field which, to some extent, compensates the effects described above.
The net radiation of the stubble field (184 Wm-2)was much lower than net radiation of sugar beet (270 Wm-2)mainly due to much higher reflection of solar radiation, as expressed by the albedo. This difference was much lower on a cloudy day (Tab.4). The active surface of the sugar beet field used near 4 times more energy on a sunny day and 3 times more on acloudy day for evapotranspiration than did stubble field. But stubble field used 2.5 times more energy for air heating than sugar beet field on a sunny day. On a cloudy day the stubble field warmed up the air while the sugar beet field was cooling it.
Generally, it can be stated that biologically active ecosystems are damping down vertical exchange of sensible energy between the earth and the atmosphere, while the biologically inactive ecosystems (bare soil, stubble field) are the factors intensifying these processes.
2. Land use changes and their effects on natural resources in agricultural landscape
Many errors in management of agricultural landscape which have been made particularly during the last century brought about many threats in the landscape. The most important are increasing water deficit, soil degradation, erosion, water pollution and impoverishment of biodiversity.
Over decades, wrong guidelines in melioration practices focused mainly on drainage of wet soils and reduction of small water bodies in the landscape, neglecting accumulation of water within catchments, and caused a very deep water deficit. This deficit was enhanced by soil and habitat degradation. One can observe the worsening of moisture conditions of grasslands in Poland (Fig. 3).Compaction of soil by heavy machines as well as decreasing organic mater content in the soil brought about worsening of water capacity and water retention of landscape. Drying up the soil together with cutting out shelterbelts and shrubs and fill up midfield ditchescaused intensification, sometimes brought to sandstorm (Fig. 4). Farmers used a lot of fertilizers, usually more than the soil capacity and more than plants could use. Not utilized fertilizers were leached into ground water (especially in lightsoil) and caused very high pollution of water (Fig. 5). Aspiration of farmers to very high yields caused simplification of crop rotation and plant cover structure, which resulted in the decrease of flora and fauna of the agricultural landscape (Fig.6).Monoculture leads to possible short-time income maximization, but adverse long-term effects as compared to heterogeneous landscape (different crops, but also islands and rows-shelterbelts of woody vegetation, strips of meadows, bushes etc, Fig. 7).
Natural land resources are being degraded through soil erosion, salinization of irrigated areas, dryland degradation from overgrazing, over-extraction of ground water, growing susceptibility to disease and build-up of pest resistance favored by the spread of monocultures and the use of pesticides, and loss of biodiversity and erosion of the genetic resource base when modern varieties displace the traditional ones.
The effects of land cover on microclimatic conditions (temperature, moisture, wind speed and so on) are well known. But the feedback of those modifications on the mesoscale air circulation, cloud formation and precipitation are less recognised. This information is crucial for tying up microscale modifications with global circulation models. Stohlgren et al. (1998) provided data indicating that land-use practices in the plains of Colorado influence regional climate and by this way they influence indirectly the vegetation in adjacent areas of the Rocky Mountains.
3. Climate variability and change and their effects on natural resources in agricultural landscape
Despite the climatic changes in the Second Millennium (Medieval Optimum and the Little Ice Age), climate was typically assumed to be nearly stable, albeit subject to high natural variability. Such climatic variables as temperature or precipitation deviated from mean value (which was considered constant over a longer period). However, nowadays one cannot really consider climate to be stable and its variability to be stationary. Stationarity is dead, as phrased by Milly et al. (2008).
There has been an increasing body of evidence of discernible ubiquitous global warming at a range of scales. As noted in IPCC (2001), the temperature increase over the 20th century for the Northern Hemisphere is likely to have been greater than for any other century in the last thousand years. Global mean surface temperature has risen by 0.65°C over the last 50 years (IPCC, 2007) with disproportionately large warming in high latitudes of the Northern Hemisphere. Twelve of the last thirteen years belong to thirteen globally warmest years on record, i.e. since 1850. Most of the observed increase in global mean air temperature since the mid-20th century is very likely due to the intensification of the greenhouse effect caused by the man-induced increase of concentrations of greenhouse gases in the atmosphere.
Observed and predicted climate changes will increasingly influence processes and threats mentioned above. Increasing air temperature together with increasing net radiation will cause increasing saturation water deficit in the atmosphere (Fig.6). This will lead to big increase of potential and real evapotranspiration, mainly in winter time.As indicated by Kundzewicz et al. (2008), shift in winter precipitation from snow to rain, and likely winter precipitation increase as temperatures rise, leads to increase of surface runoff and reduction of soil water storage in many regions. The spring snowmelt-caused runoff peak is brought forward or eliminated entirely, and winter flows increase. This, together with increasing winter evaporation, will reduce the possibility of replenishsoil water storage, leading to increase of frequency of a dry period in the summer and reduction of farmer crop yields. On the other hand, increasing frequency of extreme precipitation will lead to water erosion of soil. Decreasing actualevapotranspiration in summer period will causereduction of latent heat flux. So, more energy remains for air heating the air. The kinetics of atmosphere will lead to increase of wind speed and of the frequency and intensity of storms and tornados and, in consequence, the frequency of wind erosion. The amount of energy needed for evaporating of one-millimeter water layer can heat a layer of air of 33 m thickness by 60oC.Increasing temperature and precipitation extremes will damage the plants and small animals. Decreasing ratio of summer to winter precipitation (Fig. 7) and process of aridification causedthat climate conditions in Poland become similar to conditions of the Mediterranean region. Such process is called mediterranization. These changes are not favorable for native flora and fauna and will cause affluence of invasive species plants and animals. Also occurrence of new pests, fungi, diseases and weeds is expected. Altogether, this will worsen the biodiversity of agricultural landscapes.
Distribution of climate change impacts on agriculture shows that there will be losers and winners. Aggregate indicators show that average productivity may increase up to the global warming of 2-3oC, along with associated carbon dioxide (CO2) increase and rainfall changes, and decrease for higher warming. However, even small warming would worsen the situation for developing countries, with yield reductions at lower latitudes, and increases in numbers of people at risk of hunger. Globally, there should be major gains of potential agricultural land by the 2080s, particularly in North America (20-50%) and the Russian Federation (40-70%). However, substantial losses (up to 9%) are predicted for sub-Saharan Africa, due to the increase in drought frequency.
Agriculture in Europe is temperature-limited in the North and North-East and moisture-limited in the South and South-East. Climate change is likely to reduce the former limitation and to exacerbate the latter. However, it is likely that in the forthcoming decades the average aggregate impact of climate-related change on agriculture in Europe will be positive. Projections show a considerable increase in the area suitable for grain maize production in Europe by the end of the 21st century. Gains in agricultural area and extension of the length of the growing season are expected in the North, but shrinking of agricultural area is likely in the South of Europe. However, even small warming and reduction in precipitation jeopardize crop yield in the South of Europe, where disadvantages are likely to be predominant. Large displacement in agricultural production is expected. Some warmer season crops that currently grow mostly in southern Europe (e.g., maize, sunflower and soybeans, grapes, olive trees) will move northwards and become viable further north or at higher-altitude areas in the south. Some energy crops (e.g. rape oilseed), starch crops (e.g., potatoes), cereals (e.g., barley) and solid biofuel crops (such as sorghum and Miscanthus) are projected to expand northwards but a reduction in southern Europe is likely. Attention: if winter rainfall rises, so does nutrient leaching (e.g. in otherwise beneficially affected Scandinavia).
Projected changes in the frequency and severity of extreme climate events (e.g., spells of high temperature, droughts and intense precipitation) will have significant and adverse, consequences for food and forestry production, and food insecurity. They are expected to reduce average crop yields and livestock productivity beyond the impacts due to changes in mean variables alone, creating the possibility for surprises. Excess heat and lack of water in sensitive phases of plant development (e.g. during the anthesis of wheat) drastically reduces the crop yield. On the other hand, abundance of water (e.g. flooding of a field) or prolonged precipitation also adversely affect crops and enhance water-borne soil erosion.
Increasing climate variability will lead to increase in yield variability and will influence the risks of fires, and pest and pathogen outbreaks, with negative consequences for food, fiber and forestry.
Europe experienced a particularly extreme climate event during the summer of 2003, with temperatures up to 6°C above long-term means, and precipitation deficits up to 300 mm. Crop yield dropped by up to 20% and more in much of Southern Europe. The uninsured economic losses for the agriculture sector in the European Union were estimated at €13 billion (Easterling et al., 2007).
Rising atmospheric CO2 concentration, lengthening of the growing season due to warming, nitrogen deposition and changed management have resulted in a steady increase in annual forest CO2 storage capacity and increase of global net primary production and biomass. Thus, the overall trend towards longer growing seasons is consistent with an increase in the ‘greenness’ of vegetation, reflecting changes in biological activity (Easterling et al., 2007). However, the warming can also change the disturbance regime of forests by extending the range of some damaging insects. Increasing temperatures may increase the risk of supporting the dispersal of insects, enhancing the survival from one year to the next; and improving conditions for new insect vectors that are now limited by colder temperatures. Existence of multiple stresses, such as limited availability of water resources, loss of biodiversity, and air pollution, lead to increase in the sensitivity to climate change and reduction of resilience in the agricultural sector.
Reaction of forest ecosystems to climate change is a special problem. Dividing the world into large bio-geographic regions corresponds to climate zones – climate variables, like temperature and rainfall, create natural boundaries for species distribution. That is why it is quite obvious that changes of current temperature, humidity and rainfall characteristics, will most certainly affect plant species distribution. This hypothesis is supported by palaeo-botanical and eco-physiologic research, extensive ecosystems observation and computer simulation.