Download the Global Cooling Science Dossier (pdf, 7 MB). This includes a detailed summary of the scientific research, and 112 references to peer-reviewed scientific papers.
In dry regions that have a short rainy season, rainwater harvesting helps increase soil moisture. Increases in soil moisture bring increased growth of plants and trees. This increases cloud formation.
Low altitude clouds reflect sunlight back out into space - currently 77W/m2 on average - which makes the planet cooler. (It's true that clouds at night have an insulating effect, but much less than is reflected by daytime clouds.) Secondly, deep convective clouds carry heat energy from the land to the upper atmosphere (currently 78W/m2), from where it is radiated into space (currently 235W/m2).
NB Neither cooling process involves the Greenhouse Effect.
While the global cooling effect is important, actual impact on human beings and ecosystems is determined more by local and regional change, which sometimes act counter to global trends. So any actions at the regional level creating regional climatic improvement and decreasing climate vulnerability is worthwhile, whether or not there is a large global impact.
The strongest regional effect is increased rainfall, which helps both farmers and natural vegetation.
The global cooling effect begins as soon as the cloudiness increases, unlike the effect of CO2 emissions working via the greenhouse effect, which develop over 50+ years. This means that increased low altitude cloud cover can have an immediate and sustained cooling effect.
To help this cooling, it is necessary to work on a big enough scale, and concentrate initial efforts in certain parts of the tropics where conditions are particularly favourable. Early indications are that the most favourable areas are semi-arid zones which have a rainy season between 300 and 1000mm, e.g. Mali, Nigeria and Senegal. In some cases some of the work may already be underway or completed, as part of agricultural improvement programmes.
For maximum impact on cloudiness, studies indicate that working in swathes of land 10km long in a chess board or fishbone pattern can be particularly effective.
A Slovak hydrologist (Michal Kravcik) has proposed rainwater harvesting and water management across entire river basins in the tropics, starting with selected river basins in Africa.
This can be combined with special planting techniques developed by the Permaculture Institute of Australia to make a significant difference to soil moisture, making tree/plant survival much more likely, increasing groundwater and leading to positive feedback from shading, improved percolation and cloud formation, and potentially also from increased rainfall and cooling due to latent heat factors.
Rainwater harvesting is being promoted by the UN (UNEP) especially in Eastern and Southern Africa and so there is now a large body of experience here and in Brazil, India and China.
Nineteen senior scientists have published a call for land-atmosphere interactions to be taken into account in climate change policy. (An extract is appended.) In his comprehensive review of land-atmosphere processes, Professor Roger Pielke Snr states:
"An important conclusion from such studies is that land-use change directly alters local and regional weather and climate in two ways. First, the local and regional atmospheric conditions are changed since ... the surface heat, moisture, and other trace gas and aerosol budgets are altered.
".... vegetation and soil processes and change directly affect the atmosphere and climate globally on a variety of time and space scales. This alteration in fluxes directly modifies the environment for thunderstorms which are an effective conduit for heat, moisture, and momentum to higher latitudes, landscape processes exert a major influence on global weather and climate."
Consensus is established on the cooling role of low-altitude clouds, the ability of soil moisture and vegetation to produce increased cloud and rain, and the ability of rainwater harvesting to produce an increase in soil moisture and tree survival and growth.
To show that these three relationships work together to produce a global cooling effect is more difficult as so many other things are going on: models, though, are proving increasingly reliable and sophisticated and the accompanying science dossier provides studies showing the global impact of soil and vegetation changes in the tropics.
The regional cooling impact is easier to prove as changes have been observed over recent decades and different regions can be compared to each other. Deforestation is particularly strongly linked to regional decreases in rainfall.
Several senior UK scientists (e.g. Professor Peter Cox from the UK Meteorological Office and Dr Richard Harding of the Centre for Ecology and Hydrology) specialising in land-atmosphere interactions have been approached and have backed the project in writing - i.e. they are sufficiently confident about the effect to publicly advocate and support this approach to global cooling and drought reduction.
There will be considerable spin off benefits for (e.g.) Ethiopia, like better conditions for agriculture, reduced flooding, employment creation and less drought. Nevertheless, it is suggested that this is seen neither as an "aid" project nor an indigenous development project, but as a straight business deal - individuals, NGOs and countries wanting global cooling "pay" low income countries, workers and communities to undertake this work, partly in cash and partly in community and health projects. (Since the project will lead to an increase in pools of water, increased malaria is likely to be a downside for local people if there is no concerted effort to minimise this.)
It's nice to think that (e.g.) Ethiopia plays a lead role in saving the world, rather than "rest of the world helps poor Ethiopia with another famine".
Perhaps because of the separation between the hydrology and meteorology professions, and because climatologists are mainly concerned with understanding and describing what is happening, and warning about dangers of current human activity, rather than proposing pro-active solutions. Perhaps also because so much attention has gone on CO2 and methane, it has been easy to neglect the role of water vapour and clouds. Also, until recently this role has been hard to quantify and incorporate into climate models.
Rainwater harvesting (RWH) involves lots of medium, small and micro-scale community level projects to catch and hold rainwater in the soil during rainy periods.
There are broadly speaking 5 types of RWH: