The Working Group III of the United Nations Intergovernmental Panel on Climate Change (IPCC) issued, in May 2011, a Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN).  The report presents an assessment of the literature on the scientific, technological, environmental, economic and social aspects of the contribution of six renewable energy (RE) sources to the mitigation of climate change.  The work for the report was done by 120 researchers and it was approved by government representatives from 194 nations.

Over 150 scientific scenarios of how RE could affect energy supply by 2050 were considered and four were evaluated in depth.  Scenarios can include qualitative narratives, but this report focused only on quantitative modeling:  “They provide a plausible description of how the future may develop based on a coherent and internally consistent set of assumptions about key driving forces (e.g., rate of technological change, prices) and relationships (IPCC, 2007). In the context of this report, scenarios are thus a means to explore the potential contribution of RE to future energy supplies and to identify the drivers of renewable deployment.”  See Section of the full Report.  For the role of RE in climate change, the scenarios make various assumptions on factors such: developments in RE technologies and their costs; comparative attractiveness of competing mitigation options (e.g., nuclear energy and fossil energy with carbon capture and storage (CCS)); fundamental drivers of energy services demand (including population, economic growth); the ability to integrate variable RE sources into power grids; availability of fossil fuel resources; and others. (Note 1)       

At one level, there may not be many surprises in the analysis and conclusions of the report but at another level the report provides evidence-based support for the behavioral and policy changes for the expanded uses of renewable sources of energy that are critical if we are going to manage climate change as best as we can.  It offers an impressive estimate of what savings in greenhouse gas (GHG) emissions are possible through reliance on renewable sources of energy.

The report also reinforces the sense that climate change mitigation and adaptation are not, and have not been for some time, a hurdle for science or technology but one for political will and public policies.

The Report

The bottom line, and crowning conclusion, of the report is that, assuming the most optimistic of the scenarios, “Close to 80 percent of the world‘s energy supply could be met by renewables by mid-century if backed by the right enabling public policies.”  If that were to happen, we would save 220—560 gigatonnes (Gt) of carbon dioxide equivalent (CO2eq) between 2010 and 2050, cutting greenhouse gas (GHG) emissions by 1/3 of what they would be if business proceeds as usual.  That level of reductions could keep GHGs at 450 ppm and hold increases in global temperature below 2ºC, the stated aim of global climate change negotiations. (Note 2)  More than half of the scenarios estimate that at least 27% of primary energy supply can be met by RE in 2050.

This projection is in comparison with 2008 when of the total 492 Exajoules (EJ) (Note 3) of primary energy supply, 13%, or 64 EJ, was provided by RE, whereas oil supplied 34.6%, coal 28.4%, and gas 22.1%.  The largest RE contributor was biomass (10.2%), with the majority (roughly 60%) being traditional biomass used in cooking and heating applications in developing countries, followed by hydropower (2.3%), and other RE sources (0.4%).  In contrast to primary energy supply, in 2008 RE contributed approximately 19% of global electricity supply (16% hydropower, 3% other RE); biofuels contributed 2% of global road transport fuel supply; and traditional biomass (17%), modern biomass (8%), solar thermal and geothermal energy (2%) together fuelled 27% of the total global demand for heat. Of course, the contribution of RE to primary energy supply varies substantially by country and region.

So RE is responsible for 13% of total energy, 19% of electricity, and 27% of heat.  It is important to keep these distinctions between uses of RE in mind — primary energy, electricity, heat —when listening to arguments about the contributions of RE.

The logic behind the analysis is simple and compelling:  demand for energy is increasing, and will continue to increase; emissions from energy facilities is a significant contributor to GHGs; consumption of fossil fuels accounts for the majority of global anthropogenic GHG emissions; there are possible options for lowering GHG emissions from the energy sector, such as energy conservation and efficiency, fossil fuel switching, renewables (RE), nuclear and carbon capture and storage (CCS).  Based on these premises, it is concluded that RE can contribute to social and economic development, energy access, secure energy supply, and reduction of negative impacts on the environment and health, if implemented properly.  Of course, what does it mean to implement properly.  How do we find ways and the will to adopt policies that will attract the necessary increases in investment in technologies and infrastructure to develop RE sources.

The report analyses six renewable energy technologies to determine what their current use and what their potential is, and what policy support is required to maximize that potential.


These REs include biomass feedstocks, including forest, agricultural and livestock residues; short-rotation forest plantations; energy crops; the organic component of municipal solid waste; and other organic waste streams.  They can be used to produce electricity or heat or some forms of fuels and the output is generally constant or controllable.  The report indicates that such REs are important for traditional cooking and heating in developing countries and account for about 10% of global energy supply, but are likely to decline in future decades. In addition, converting land into agricultural biomass and energy crops can generate more GHG than they save.

Direct Solar Energy

Included in this category of REs are photovoltaics (PV) and concentrating solar power (CSP) that produce thermal energy (heating or cooling, either through passive or active means), provide direct lighting needs and, potentially, produce fuels that might be used for transport and other purposes.  At present they produce a fraction of 1% of total global energy supply and have the most room for growth with the potential, depending on the scenario, to contribute from 10% to 33% of global electricity generation by 2050.  That growth will come only from innovations, cost reductions and public policies.  Solar energy is variable and, to some degree, unpredictable, but in some circumstances and at certain times it correlates relatively well with energy demands.

Geothermal energy

Such form of energy utilizes the accessible thermal energy from the Earth’s interior with heat extracted from geothermal reservoirs using wells or other means. When used to generate electricity, geothermal power typically offers constant output.  Currently it makes a minor contribution to energy but it is expected to increase to about 3% of global electricity demand and 5% of heat demand.


Harnessing the energy of water moving from higher to lower elevations is used primarily to generate electricity.  Hydropower projects encompass dam projects with reservoirs, run-of-river and in-stream projects and can offer energy on a large scale, for centralized urban communities, or on smaller scales for dispersed rural communities.  While there is some temporal variability of supply, reservoirs counter this factor.

In 2008 it provided 16% of global electricity supply, the largest of all the RE sources for electricity.  This source is actually estimated to decrease to about 10-14% of electricity supply, only because while there will be more hydropower, energy demand and electrification will expand resulting in a lower share for hydropower.

Ocean energy

The energy from oceans comes from the potential, kinetic, thermal and chemical energy of seawater, which can be transformed to provide electricity, thermal energy, or potable water. These REs are largely at the developmental stage and their reliability or predictability are unclear.

Wind energy

Such energy harnesses the kinetic energy of moving air to produce electricity from large wind turbines located on land (onshore) or in sea- or freshwater (offshore).  Wind turbines and farms are developed technologies and the electricity generated is both variable and, to some degree, unpredictable, but experience and detailed studies from many regions have shown that the integration of wind energy generally poses no insurmountable technical barriers.

As of 2009, wind power capacity installed provided 2% of worldwide electricity demand.  This RE is expanding rapidly in Europe, North America, and China.  Under certain scenarios wind power can reach a 20% share of energy by 2050.

RE and Integration, Sustainable Development, and Mitigation Issues

Developing the technologies of RE is one issue.  Getting the energy from those RE sources into the energy grid, and into end-users (homes, buildings and other facilities) is another matter.  The report concludes that integrating RE into most existing energy supply systems and end-use sectors at an accelerated rate—leading to higher shares of RE—is technologically feasible, though it does present a number of challenges.  Storage of unpredictable RE sources, such as wind and solar and some hydropower, is one such challenge.

RE can be integrated into all types of electricity systems, from large inter-connected continental-scale grids down to small stand-alone systems and individual buildings.  This explains why electricity is expected to attain higher shares of RE earlier than either the heat or transport fuel sectors at the global level.

Economic development has been associated with increased energy use, and resulting GHG emissions, and the report supports the possibility of decoupling development and GHG emissions through RE.

Access to energy, without further polluting the planet, is particularly critical for many developing countries. Many developing countries already have acquired expanded energy supply through RE fed into both decentralized and centralized grids, and further opportunities exist for modernizing energy services, for example, by using solar energy for water heating and crop drying, biofuels for transportation, biogas and modern biomass for heating, cooling, cooking and lighting, and wind for water pumping.  It is expected that over the long term RE will be deployed at higher levels in developing countries than in developed countries.

Some Concluding Points

Studies have consistently found that the total global technical potential for RE is substantially higher than both the current and projected future global energy demand, with solar having the highest potential.  In other words, there is enough RE out there to meet energy demands, now and in the future.

The report found that despite all the variations in assumptions, …”the scenarios indicate that, all else being equal, more ambitious mitigation generally leads to greater deployment of RE.”  That is, the more ambitious the target for reducing GHG emissions, the higher the use of RE.

Increasing the installed capacity of RE power plants will reduce the amount of fossil and nuclear fuels that otherwise would be needed in order to meet a given electricity demand.  Part of the answer is to combine different variable renewable resources from larger geographical areas.  The developing and proposed interconnectors between electric grids in the island of Ireland and Britain and between Britain and the Continent are steps in this direction.  Wind, in particular, needs to be distributed to wider geographical areas, beyond Europe, North America, and China.

In most regions of the world, policy measures are still required to ensure a more rapid deployment of many RE sources. Policies include regulations such as feed-in-tariffs, quotas, priority grid access, building mandates, biofuel blending requirements, and bioenergy sustainability criteria.   Other policy categories include fiscal incentives, such as taxes, rebates and grants; and public finance mechanisms such as loans and guarantees. Wider policies aimed at reducing GHG emissions such as carbon pricing mechanisms may also support RE.  Policies can be implemented within sectors of the economy and/or at different geographical levels — local, state/provincial, national, regional and global.

Perhaps the fundamental policy change that is required is that emissions of GHGs and other pollutants from energy sources, and elsewhere, must be given a price that is included in the costs of energy.  Part of that price should be the costs of air pollution and impacts on public health.


(1) The discussion of scenarios is found in Section 10 of the full report.  A brief description of the use and basis for scenarios would have been helpful in the “Summary for Policymakers.”

(2) Of course some opine that we must hold GHGs at well below 450 ppm in order to avoid drastic changes to the climate.  See, e.g.,

(3) The report uses the Exajoule as the measure of energy.  1 Exajoule = 1018 joules = 23.88 million tonnes of oil equivalent (Mtoe).


IPCC, 2011: “Summary for Policymakers.” In: IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.   A 25-page Summary, for policy advisors, is provided in addition to the 1,000 page report.

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