The geographic distribution of plant species, vegetation types and agricultural cropping patterns demonstrate the very strong control that climate has on plant growth. Solar radiation, temperature and precipitation values and seasonal patterns are key determinants of plant growth through a variety of direct and indirect mechanisms. Other climatic characteristics are also major influences, such as wind speed and storm frequency. There is a rapidly growing number of well-documented instances of change in ecosystems due to recent (and probably anthropogenic) climate change (Walther et al., 2002). For example, there are several lines of evidence in the Arctic, ranging from indigenous people's knowledge to satellite images, that show that species distributions have changed, with growing shrub cover and increasing primary productivity (Callaghan et al., 2004). Another example is that plant species composition in the mountains of central Norway has changed over a 70-year period, with lowland species coming in and snow-bed and high-altitude species disappearing (Klanderud & Birks, 2003). Meta-analyses of data for well-studied alpine herbs, birds and butterflies by Parmesan and Yohe (2003) found a mean range shift of approximately 6 km per decade towards the poles or 6 m per decade in elevation, and that the date of the start of spring has advanced by 2 days per decade. In agriculture, there are clear examples of recent climate change affecting plant growth and cropping potential or performance. For example, in Alberta (Canada) the potential maize-growing zone, defined by temperature limits, has shifted north by 200-300 km over the last century (Shen et al., 2005). However, climate change is not just affecting temperate zones. For example, in some arid zones there have been increases in precipitation, leading to increased shrub density, and changes in the rest of the ecosystem (e.g. Brown et al., 1997). Overall, the Intergovernmental Panel on Climate Change (IPCC, 2001b) concluded that 'from collective evidence, there is high confidence that recent regional changes in temperature have had discernible impacts on many physical and biological systems'. These recent climate changes are likely to accelerate as human activities continue to perturb the climate system, and many reviews have made predictions of serious consequences for ecosystems (e.g. Izaurralde et al., 2005) and for food supplies and food security (e.g. Reilly et al., 2003; Easterling & Apps, 2005). This chapter outlines recent past and future anthropogenic climate change. Much of the relevant research has already been drawn together, reviewed and summarised by the many contributors to the IPCC
reports (IPCC, 2001a-c), and we have therefore relied heavily on that authoritative source of information.
The recent and future anthropogenic changes to the climate have to be considered in the context of natural climate changes. The Earth's climate results from the complex interaction of many components: the ocean, atmosphere, geosphere, cryosphere and biosphere. Although the climate system is ultimately driven by the external solar energy, changes to any of the internal components, and how they interact with each other, as well as variability in the solar radiation received can lead to changes in climatic conditions. These influences are often considered as 'forcings', changes to the energy inputs and outputs that result in modifications in the climate. Therefore there are many causes of climate change that operate on a variety of timescales. On the longest timescales are mechanisms such as geological processes and the changes in the Earth's orbit around the sun (Milankovitch-Croll effect). The latter is believed to be the mechanism underlying the cycle of ice ages and interglacials. Geological processes resulting from the movement of tectonic plates and consequent major changes in physical relief, continental distributions and ocean basin shape and connectivity clearly have influenced global climate patterns. Geological processes can also work on a much shorter timescale through volcanism. Large, explosive volcanic eruptions can inject millions of tons of soot and ash into the middle atmosphere where they reflect solar radiation, creating a 'global soot veil'. The Tambora eruption in Southeast Asia in 1815 caused extensive global cooling and 'the year without a summer' in Europe (e.g. Engvild, 2003; Oppenheimer, 2003). The climate impacts of such volcanic events usually decay after 1 or 2 years (as in the Mt. Pinatubo eruption of June 1991, which caused 0.25-5°C drop in mean temperature for 1-2 years in several parts of the world; Hansen et al., 1996). However, some research has suggested that very infrequent, regional so-called supereruptions can alter the climate for enough time to cause radical species loss (Rampino, 2002), although this is much debated.
In addition to geological and orbital changes, the climate system is sensitive to inherent and periodic internal variability in any one of its components, such as ocean currents. These can be on decadal timescales, such as the Interdecadal Pacific Oscillation. Or the variations can be on near-interannual timescales, such as the well-documented El-Nino/Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO). During ENSO events when the ocean upwelling in the eastern equatorial Pacific is weaker than normal, the resulting changes to sea surface temperatures and to the wind patterns dramatically affect climate and consequently impact the biosphere across the region. For example, in El Nino years, maize yields in China are decreased by 5% (Tao et al., 2004) and in Australia wheat crop yields are closely related to the severity of rainfall reductions (e.g. Nicholls, 1985). The NAO has wide ecological effects (e.g. reviewed by Ottersen et al., 2001), such as determining the length of the growing season in Europe, as evident in extensive phenological observations (see Chapter 4, Menzel, 2003). Also correlations of the NAO index have been found with crop yields in Europe and North America (e.g. Gimeno et al.,
2002). The overall effects of such internal changes on climate are difficult to predict, because of the feedbacks between the climate system components. For example, an ocean current change might warm a high-latitude region, leading to reduced snow cover, which in turn leads to more land surface exposure and more solar energy absorption which results in a positive feedback.
Although most public discussion on climate change currently focuses on fossil fuel combustion, CO2 emissions and the enhanced 'greenhouse effect', it must be noted that there are other components of human-induced climate change. Human activity has modified, and continues to modify, the Earth's surface on a very large scale, through deforestation, afforestation, cultivation, mineral extraction, irrigation, drainage and flooding. These large alterations in land cover change the surface shortwave reflectivity and hydrological and thermal properties of the land surface. Thus, replacing forest with pasture changes the surface energy balance and increases the proportion of radiant energy going into heating the air and reducing evaporation, as many studies have shown (e.g. von Randow et al., 2004). Conversely, the very large expansion of irrigation in previously dry areas changes land cover and solar radiation absorption and increases energy partitioning into evaporation, as well as changing the seasonal pattern of surface-atmosphere exchanges (e.g. Adegoke etal.,
The crux of the enhanced greenhouse effect is that human modification of the atmospheric concentration of the key radiation-absorbing gases - CO2, CH4, N2O and various halocarbons - has resulted in a radiative forcing of the climate system. These gases have been released primarily as a result of industrial, transport and domestic activities and to a lesser extent from agricultural activities and land use changes (IPCC, 2001a). Direct and indirect determination of CO2, CH4 and N2O in the atmosphere over the past 1000 years show marked and unprecedented increases in concentrations in recent times (Figure 1.1). The start of these increases coincides with the rapid industrialisation of the Northern Hemisphere during the late eighteenth and nineteenth centuries, and so since 1750, the global mean atmospheric concentration of CO2 has increased by 31%; approximately 75% of this increase has come from fossil fuel combustion and 25% from land use change (IPCC, 2001a). Analysis of extended data sets from ice cores indicates that the current atmospheric concentration of CO2 is the highest for the past 420 000 years, and is likely to be the highest within the last 20 million years (IPCC, 2001a). The percentage increase in methane concentrations is greater, having risen by 151% since 1750, whilst the concentration of nitrous oxide has increased by 17% over the same period (IPCC, 2001a). The estimated radiative forcing associated with the increased concentrations
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