Radiocarbon dating and U Th ratios

Of the various radiometric methods available, radiocarbon dating has been by far the most useful for organic material formed within the last 40 000 years. Radioactive 14C is produced in the atmosphere by cosmic bombardment. It is incorporated in living organisms and, on the death of the organism, provided it is excluded from exchange with the atmosphere, the 14C decays in accordance with its half-life of 5730 years. Its decay thus constitutes a radiometric clock whereby determination of the...

Changed global nutrient cycles

We have already seen in the previous chapter that atmospheric concentrations of N2O have risen over the last two centuries largely as a result of fossil fUel combustion, but this is not the only, or indeed the most significant disruption of the nitrogen cycle as a result of human activities. One of the most remarkable trends during the course of the twentieth century has been the relentless increase in the extent to which anthropogenic processes have begun to dominate the conversion of...

Coastal and marine impacts

Many of the processes described above in this and the previous chapter have important implications for coastal and marine ecosystems. One consequence of the changes to the nitrogen and phosphorus cycles has been a massive increase in the flux of these nutrients to many coastal regions and near-shore marine environments. This process has been reinforced by the massive increase in the number of people living in coastal regions and generating organic waste, much of it for ultimate disposal in the...

Iron fertilisation experiments and the biological pump

The question of iron fertilization and ocean productivity - one of the key elements in the above hypothesis - has been addressed through direct experiments in areas of the ocean where productivity is low despite high nitrate concentrations in the water. Figure 6.8 A schematic diagram of the feedbacks in the climate system involved in the hypothesis of iron fertilisation activating the 'biological pump' in the oceans. At the onset of glaciation, lower sea-levels, drier climatic conditions and...

Changes in lakewater chemistry

Eutrophication can be the outcome of natural processes and many lakes are naturally eutrophic. Cultural eutrophication is the chemical enrichment of the water body through human impacts on nutrient supplies. It generally leads to increased productivity, hence higher quantities of organic carbon, which can result in major changes in the structure and functioning of the lake ecosystem, especially where the decomposition of the additional carbon involves high levels of oxygen demand by decomposer...

Defining and exploring the key questions

Any of our ancestors living a full three score years and ten in Western Europe some 11 600 years ago would have experienced, during their life time, truly remarkable changes in climate. Evidence from that time shows that the main changes took place over a period of 50 years at most. Although different lines of evidence give different figures for the degree of warming, it would be difficult to argue for an increase of less than 4 C in mean annual temperature over much of Western Europe. In many...

Future droughts

The various sections of text above bring us closer to a consideration of future climate change in relation to human welfare. This is the main focus of the next chapter. This section, by briefly considering water scarcity, begins to build a bridge towards a more rounded view of the future impacts of all aspects of global change. It is important to bear in mind distinctions between meteorological (deficit of precipitation), agricultural (largely soil-moisture deficit) and hydro-logical (low...

Domain teleconnections and global impacts

The generation of strong atmospheric convection, linked to the maintenance of high SSTs in the area of the IPWP, is but one of the processes ensuring that ENSO variability has a widespread impact on climates and environmental processes well outside the tropical Pacific region. These include the extent of Antarctic sea-ice, Atlantic-ocean circulation and the inter-annual variations in atmospheric-CO2 concentrations. There is evidence to suggest that variability in the tropical Pacific leads...

Dating recent sediments

Many human impacts on environmental systems have accelerated and become more widespread over the last two centuries and especially the last 50 years. In the case of climate, instrumental records are available for this period, but the 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 -I 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 -I Figure 3.6 Correlations based on changes in palaeomagnetic secular variation PSV as recorded in lake sediments. The upper row of graphs shows changes in...

Increasing concentrations of greenhouse gases

Raynaud Anthropogenic Increase

By combining measurements from ice cores with direct measurements from 1958 onwards, it is now possible to reconstruct the history of the main Figure 8.2 Atmospheric trace-gas and sulphate trends over the last 1000 years compiled from ice-core records and direct measurements. The gas measurements are all derived from ice-core and firn measurements, except for the latest CH4 open inverted triangles and CO2 values solid line . Modified from Raynaud et al., 2003. atmospheric greenhouse-gas...