The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources – their availability, quality, use and management. It takes into account current and projected regional key vulnerabilities, prospects for adaptation, and the relationships between climate change mitigation and water. Its objectives are:

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Climate change and water.

This study analyzed determinants of farm-level climate adaptation measures in Africa using a multinomial choice model fitted to data from a cross-sectional survey of over 8000 farms from 11 African countries. The results indicate that specialized crop cultivation (mono-cropping) is the agricultural practice most vulnerable to climate change in Africa. Warming, especially in summer, poses the highest risk. It encourages irrigation, multiple cropping and integration of livestock. Increased precipitation reduces the probability of irrigation and will benefit most African farms, especially in drier areas. Better access to markets, extension and credit services, technology and farm assets (labor, land and capital) are critical for helping African farmers adapt to climate change. Government policies and investment strategies must support education, markets, credit and information about adaptation to climate change, including technological and institutional methods, particularly for poor farmers in the dry areas of Africa.

Determinants of African farmers’ strategies for adapting to climate change: multinomial choice analysis

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Ecosystems, their properties, goods and services

An assessment of the contribution of plant physiological effects to future changes in continental water runoff suggests that flooding risk under future global warming scenarios may be greater than was assumed. The stomatal pores that allow CO2 to enter plants and water to escape open less widely when CO2 concentrations are high, reducing water loss from the plant and thus leaving more water at the land surface. This effect may have contributed to the increase in continental runoff observed during the twentieth century, but most predictions of future changes in runoff don't account for it. The concept of 'CO2 equivalent', widely used to compare the effects of greenhouse gases on climate, does not account for this effect, so it may need to be revisited in light of these findings. An ensemble of experiments is used with a global climate model to assess the contribution of plant 'physiological forcing' to future changes in continental runoff. It is found that the effect increases simulated global mean runoff by 6 per cent when the concentration of carbon dioxide is doubled relative to pre-industrial levels; an increase that is comparable to that simulated in response to climate change caused by radiative forcing. This finding suggests that the risk of flooding may be greater than previously assumed under future global warming scenarios. In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration influences the climate system through its effects on plant physiology1. Plant stomata generally open less widely under increased carbon dioxide concentration2, which reduces transpiration1,3,4,5,6 and thus leaves more water at the land surface7. This driver of change in the climate system, which we term ‘physiological forcing’, has been detected in observational records of increasing average continental runoff over the twentieth century8. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11 ± 6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing9,10,11 will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.

Projected increase in continental runoff due to plant responses to increasing carbon dioxide

The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal “antitranspirant” response of plants to rising atmospheric CO2 [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835–838]. However, CO2 also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO2 levels in a global vegetation model, we find a decrease in global runoff from 1901 to 1999. This finding highlights the importance of vegetation structure feedback on the water balance of the land surface. Therefore, the elevated atmospheric CO2 concentration does not explain the estimated increase in global runoff over the last century. In contrast, we find that changes in mean climate, as well as its variability, do contribute to the global runoff increase. Using historic land-use data, we show that land-use change plays an additional important role in controlling regional runoff values, particularly in the tropics. Land-use change has been strongest in tropical regions, and its contribution is substantially larger than that of climate change. On average, land-use change has increased global runoff by 0.08 mm/year2 and accounts for ≈50% of the reconstructed global runoff trend over the last century. Therefore, we emphasize the importance of land-cover change in forecasting future freshwater availability and climate.

https://www.pnas.org/content/pnas/104/39/15242.full.pdf

Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends.

Across Africa, perennial drainage density as a function of mean annual rainfall defines three regimes separated by threshold values of precipitation. This nonlinear response of drainage to rainfall will most seriously affect regions in the intermediate, unstable regime. A 10% decrease in precipitation in regions on the upper regime boundary (1000 millimeters per year) would reduce drainage by 17%, whereas in regions receiving 500 millimeters per year, such a drop would cut 50% of surface drainage. By using predicted precipitation changes, we calculate that a decrease in perennial drainage will significantly affect present surface water access across 25% of Africa by the end of this century.

https://science.sciencemag.org/content/sci/311/5769/1917.full.pdf

Changes in surface water supply across Africa with predicted climate change.

A proposed drainage evolution model for Central Africa—Did the Congo flow east?

Abstract The concept of South America and Africa as rigid continents during the formation, growth and motion of their respective plates has frustrated reconstruction of a tight, geologically economic fit between these two fragments in their Gondwana framework. We recognize that (1) internal strains released during and following Gondwana break-up have distorted their actual shapes within Gondwana and (2) these two continents comprise mosaics of smaller microblocks, or platelets, of relatively undistorted Precambrian terrains that experienced modest, episodic relative motions along rift zones that separate them. This permits a fresh approach to quantitative reconstructions of palaeo-continents. Former geological ties forged at the time of Gondwana amalgamation, now exposed at the continental margins of the South Atlantic as piercing points, provide robust anchors for new paleo-cartographic experiments. We present two new tectonic maps of the Brasiliano and Pan-African structures of West Gondwana on which we identify ten piercing points that, if re-joined simultaneously, could facilitate quantification of a well-substantiated Gondwana fit and help retrace the evolution of its continental margins with greater accuracy than has been achieved until now. This has significant bearing on understanding the origin and evolution of passive continental margins, and the geodynamics of Gondwana break-up.

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Restoring Pan-African–Brasiliano connections: more Gondwana control, less Trans-Atlantic corruption

[1] For the first time, ambient noise tomography is used to clearly image the magma chamber beneath Lake Toba caldera, one of the largest Quaternary calderas on Earth. Using data from 40 seismic stations deployed between May and October 2008 around Lake Toba, empirical Green's functions are extracted from long term cross-correlations of continuous records. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the period range from 2.5 to 12 seconds. Arrival times of these waves are picked for a given period and inverted using 2-D tomography to calculate lateral variations in velocity for the given period. This was done for six different periods, which all correspond to different sampling depths. Thus the six 2-D models presented together provide information on velocity variations with depth. The results show a low-velocity body coincident with the Lake Toba caldera, representing the magma chamber under the volcano. The chamber is observed to have a complex 3-D geometry, with at least two separate sub-chambers underlying the caldera. Other results include a deep low velocity body, possibly another magma chamber, south west of the lake with an upper limit of ~7 km depth. The maximum depth to which this body reaches could not be resolved. The Sumatra Fault marks a velocity contrast, but only down to depths not greater than 5 km. The reliability of the results was further confirmed by checkerboard recovery tests.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010GL044211

Lake Toba volcano magma chamber imaged by ambient seismic noise tomography

https://dspace.library.uu.nl/bitstream/handle/1874/8000/hilst_02_crustal-thickness.pdf?sequence=1

Jacek Stankiewicz

Papers

Changes in surface water supply across Africa with predicted climate change.

A proposed drainage evolution model for Central Africa—Did the Congo flow east?

Restoring Pan-African–Brasiliano connections: more Gondwana control, less Trans-Atlantic corruption

Lake Toba volcano magma chamber imaged by ambient seismic noise tomography

Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversions