The Intergovernmental Panel on Climate Change (IPCC) is a global organization established to collect and assess the scientific literature related to climate change and its environmental and socioeconomic impacts. Its Working Group III (WGIII) report is one of three that together provide a complete overview of the state of climate science. It covers pathways and scenarios for mitigating climate change. This overview pulls out notable findings from the WGIII Summary for Policymakers (SPM).
Greenhouse gas emissions are rising at rates leading to dangerous climate change. The total amount of greenhouse gases in the atmosphere produced by human activities doubled in the last 40 years. (jump to section)
Emissions are rising due to economic growth, population growth and increased coal burning. Overall, carbon dioxide from fossil fuels and industry has accounted for about ¾ of greenhouse gas emissions since 1970. (jump to section)
Steep emission cuts are needed to avoid dangerous impacts. If greenhouse gas concentrations rise too high, we’ll miss temperature targets. (jump to section)
Existing policies and pledges will need to be strengthened. Current policies (as described in the Cancun Pledges) are consistent with pathways more likely than not to exceed 2C of warming. (jump to section)
The energy sector will need to be a leader in the transition to a lower-carbon economy. Renewable energy will need to triple or quadruple by 2050. (jump to section)
Despite these challenges, restricting warming to 2C remains feasible. The cost of many renewables is falling and their use is spreading. Deforestation rates have also declined. (jump to section)
Restricting warming to 2C is also affordable. Reductions in growth would be small, especially compared to the impacts of unchecked warming. (jump to section)
Delaying efforts to curb greenhouse gas pollution will make both mitigation costs and climate impacts themselves more costly. Overshooting targets means CO2 will need to be removed from the air. (jump to section)
Effective policies will move forward while accounting for risk and uncertainty, rather than waiting for total certainty. Some risks are low-probability but high impact, and need to be taken into account. (jump to section)
Investing in long-lived fossil fuel infrastructure could lock us in to a harmful emissions pathway. These unwise investments will be difficult and costly to change. (jump to section)
Findings in Depth
Greenhouse gas emissions are rising at rates leading to dangerous climate change.
The total amount of greenhouse gases in the atmosphere produced by human activities doubled in the last 40 years, reaching an overall concentration of greenhouse gases in the atmosphere (in CO2-equivalent, CO2eq) around 430 parts per million (ppm) in 2011. This increase is has accelerated since the publication of the AR4.
At current rates, the greenhouse gas concentration by 2100 would be 750-1300ppm CO2eq – resulting in projected warming of 3.7 – 4.8 degrees C. Most available research shows that climate change impacts in this temperature range would be extremely disruptive to human society and could be catastrophic.
“About half of cumulative anthropogenic CO2 emissions between 1750 and 2010 have occurred in the last 40 years (high confidence).” [SPM pg.6]
“For comparison, the CO2eq concentration in 2011 is estimated to be 430 ppm (uncertainty range 340–520 ppm).” [SPM pg.9]
“Baseline scenarios, those without additional mitigation, result in global mean surface temperature increases in 2100 from 3.7 to 4.8°C compared to preindustrial levels.” [SPM pg.8]
“Global greenhouse gas (GHG) emissions have grown since pre-industrial times, with an increase of 70% between 1970 and 2004 (high agreement, much evidence).” [SPM pg.3]
From 2000-2010, the rate of emissions growth doubled to 2.2% per year. This acceleration has been attributed primarily to an increase in burning coal. Overall, carbon dioxide from fossil fuels and industry has accounted for about ¾ of greenhouse gas emissions since 1970. AR4 noted the role of economic and population growth, but did not call out coal as AR5 does.
“Globally, economic and population growth continue to be the most important drivers of increases in CO2 emissions from fossil fuel combustion. The contribution of population growth between 2000 and 2010 remained roughly identical to the previous three decades, while the contribution of economic growth has risen sharply (high confidence).” [SPM pg.7]
“Total anthropogenic GHG emissions have continued to increase over 1970 to 2010 with larger absolute decadal increases toward the end of this period (high confidence). Despite a growing number of climate change mitigation policies, annual GHG emissions grew on average by 1.0 gigatonne carbon dioxide equivalent (GtCO2eq) (2.2%) per year from 2000 to 2010 compared to 0.4 GtCO2eq (1.3%) per year from 1970 to 2000.” [SPM pg.5]
“In the last decade, the main contributors to emission growth were a growing energy demand and an increase of the share of coal in the global fuel mix.” [SPM pg.23]
“CO2 emissions from fossil fuel combustion and industrial processes contributed about 78% of the total GHG emission increase from 1970 to 2010, with a similar percentage contribution for the period 2000–2010 (high confidence).” [SPM pg.5]
“The effect on global emissions of the decrease in global energy intensity (-33%) during 1970 to 2004 has been smaller than the combined effect of global per capita income growth (77 %) and global population growth (69%); both drivers of increasing energy-related CO2 emissions.” [SPM pg.3]
To have a good chance of meeting the globally agreed 2C target, global CO2 concentrations should never rise above 530ppm CO2eq and should stabilise at 430-480ppm in 2100. Emissions in 2010 were 49Gt (gigatonnes) CO2eq, but to reach the target emissions should be 30-50Gt CO2eq in 2030. This builds on the findings of AR4, which noted that the next few decades will be critical in determining where we will ultimately be able to stabilize emissions.
“Mitigation scenarios reaching concentration levels of about 500 ppm CO2eq by 2100 are more likely than not to limit temperature change to less than 2°C relative to preindustrial levels, unless they temporarily “overshoot” concentration levels of roughly 530ppm CO2eq before 2100, in which case they are about as likely as not to achieve that goal. Scenarios that reach 530 to 650ppm CO2eq concentrations by 2100 are more unlikely than likely to keep temperature change below 2°C relative to preindustrial levels.” [SPM pg.11]
“Mitigation scenarios in which it is likely that the temperature change caused by anthropogenic GHG emissions can be kept to less than 2C relative to preindustrial levels are characterized by atmospheric concentrations in 2100 of about 450ppm CO2eq (high confidence).” [SPM pg.11]
“Of the 49 (±4.5) GtCO2eq/yr in total anthropogenic GHG emissions in 2010, CO2 remains the major anthropogenic GHG accounting for 76% (38±3.8 GtCO2eq/yr) of total anthropogenic GHG emissions in 2010.” [SPM pg.5]
“Cost effective mitigation scenarios that make it at least as likely as not that temperature change will remain below 2°C relative to pre industrial levels (2100 concentrations between about 450 and 500 ppm CO2eq) are typically characterized by annual GHG emissions in 2030 of roughly between 30 GtCO2eq and 50 GtCO2eq.” [SPM pg.16]
“Mitigation efforts over the next two to three decades will have a large impact on opportunities to achieve lower stabilization levels (see Table SPM.5, and Figure SPM. 8).” [SPM pg.15]
Current policies (as described in the Cancun Pledges) are consistent with pathways leading to 550-650ppm CO2eq by 2100. This outcome is more likely than not to exceed 2C of warming.
“Estimated global GHG emissions levels in 2020 based on the Cancún Pledges are not consistent with cost effective long term mitigation trajectories that are at least as likely as not to limit temperature change to 2°C relative to pre industrial levels (2100 concentrations of about 450 and about 500ppm CO2eq), but they do not preclude the option to meet that goal (high confidence). Meeting this goal would require further substantial reductions beyond 2020.” [SPM pg. 15]
Energy was responsible for 35% of global emissions in 2010. Switching from coal to gas can help if gas leaks are low, but by 2050 average energy-related emissions need to be below that for the best gas-fired plants. Projections show that in order to reach the 2C target, low-carbon energy generation (renewable, nuclear and fossil fuels with CCS) would need to increase its share of the energy mix by three to four times by 2050. Unabated fossil fuel burning would need to phase out completely by 2100. This expands and adds detail to the findings of the AR4.
“GHG emissions from energy supply can be reduced significantly by replacing current world average coal fired power plants with modern, highly efficient natural gas combined cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated (robust evidence, high agreement). In mitigation scenarios reaching about 400ppm CO2eq concentrations by 2100, natural gas power generation without CCS acts as a bridge technology, with deployment increasing before peaking and falling to below current levels by 2050 and declining further in the second half of the century (robust evidence, high agreement).” [SPM pg.23]
“At the global level, scenarios reaching 450 ppm CO2eq are also characterized by more rapid improvements of energy efficiency, a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewables, nuclear energy and fossil energy or bioenergy with carbon dioxide capture and storage (BECCS) by the year 2050.” [SPM pg.15]
“In the majority of low stabilization scenarios, the share of low carbon electricity supply (comprising renewable energy (RE), nuclear and CCS) increases from the current share of approximately 30% to more than 80% by 2050, and fossil fuel power generation without CCS is phased out almost entirely by 2100.” [SPM pg.23]
“For lower stabilization levels, scenarios put more emphasis on the use of low-carbon energy sources, such as renewable energy and nuclear power, and the use of CO2 capture and storage (CCS). In these scenarios improvements of carbon intensity of energy supply and the whole economy need to be much faster than in the past.” [SPM pg.16]
The cost of many renewables is falling and their use is spreading. Renewables accounted for over half of new capacity added globally in 2012, led by growth in wind, hydro, and solar. Deforestation rates have also declined, and emissions from deforestation and other land use changes are projected to continue to decrease. Land use change may even represent a net carbon sink by the end of the century (Pg.27). Existing policies in some countries have successfully weakened the link between emissions and GDP – growing their economies without a corresponding growth in emissions. This represents a major shift since AR4 in terms of the widespread availability and use of renewable energy.
“Since AR4, many RE technologies have demonstrated substantial performance improvements and cost reductions, and a growing number of RE technologies have achieved a level of maturity to enable deployment at significant scale (robust evidence, high agreement). Regarding electricity generation alone, RE accounted for just over half of the new electricity generating capacity added globally in 2012, led by growth in wind, hydro and solar power.” [SPM pg.23]
“Most recent estimates indicate a decline in AFOLU CO2 fluxes, largely due to decreasing deforestation rates and increased afforestation. However, the uncertainty in historical net AFOLU emissions is larger than for other sectors, and additional uncertainties in projected baseline net AFOLU emissions exist. Nonetheless, in the future, net annual baseline CO2 emissions from AFOLU are projected to decline, with net emissions potentially less than half the 2010 level by 2050 and the possibility of the AFOLU sectors becoming a net CO2 sink before the end of century (medium evidence, high agreement).” [SPM pg.27]
“In some countries, tax-based policies specifically aimed at reducing energy consumption or emissions – alongside technology and other policies – have helped to weaken the link between GHG emissions and GDP (high confidence).” [SPM pg.31]
“The range of stabilization levels assessed can be achieved by deployment of a portfolio of technologies that are currently available and those that are expected to be commercialised in coming decades.” [SPM pg.16]
The transition needed to give a good chance of keeping warming below 2C (450ppm CO2eq) would cut global economic growth from a high-end estimate of 3% per year in a world without mitigation or climate impacts, to 2.96% per year (Pg.17). By contrast, IPCC Working Group 2 concluded warming around 2.5C would cost up to 2% of GDP, but noted this was “incomplete,” depended on “disputable assumptions” and did not take account of potential “catastrophic” changes. Much of this cost is now inevitable due to past emissions, and WG2 found that without mitigation, temperatures will rise past 2.5C and losses will accelerate.
“Under these assumptions, mitigation scenarios that reach atmospheric concentrations of about 450ppm CO2eq by 2100 entail losses in global consumption – not including benefits of reduced climate change as well as co-benefits and adverse side effects of mitigation – of 1% to 4% (median: 1.7%) in 2030, 2% to 6% (median: 3.4%) in 2050, and 3% to 11% (median: 4.8%) in 2100 relative to consumption in baseline scenarios that grows anywhere from 300% to more than 900% over the century. These numbers correspond to an annualized reduction of consumption growth by .04 to 0.14 (median: 0.06) percentage points over the century relative to annualized consumption growth in the baseline that is between 1.6% and 3% per year.” [SPM pg.17]
“The total economic effects at different temperature levels would include mitigation costs, co-benefits of mitigation, adverse side effects of mitigation, adaptation costs, and climate damages. Mitigation cost and climate damage estimates at a given temperature level cannot be compared to evaluate the costs and benefits of mitigation. Rather, the consideration of economic costs and benefits of mitigation should include the reduction of climate damages relative to the case of unabated climate change.” [SPM pg.17]
Delaying efforts to curb greenhouse gas pollution will make both mitigation costs and climate impacts themselves more costly.
If the delay is serious enough, it may be necessary to use carbon dioxide removal (CDR) technology to pull CO2 from the air. CDR technology is unproven at scale, however ,and carries its own costs and drawbacks. A far cheaper and better path would be to reduce emissions now so that CDR is not needed.
“Delaying mitigation efforts beyond to those in place today through 2030 is estimated to substantially increase the difficulty of the transition to low longer-term emissions levels, and narrow the range of options consistent with maintaining temperature change below 2°C relative to preindustrial levels (high confidence).” [SPM pg.16]
Scenarios with delayed mitigation and high 2030 emissions rates exhibit “higher transitional and long term economic impacts.” [SPM pg.16]
“Delaying mitigation further increases mitigation costs in the medium to long term (Table SPM.2, blue segment).” [SPM pg.17]
“Depending on the level of the overshoot, overshoot scenarios typically rely on the availability and widespread deployment of BECCS and afforestation in the second half of the century. The availability and scale of these and other Carbon Dioxide Removal (CDR) technologies and methods are uncertain and CDR technologies and methods are, to varying degrees, associated with challenges and risks (see Section SPM 4.2).” [SPM pg.15]
“Barriers to large scale deployment of bioenergy include concerns about GHG emissions from land, food security, water resources, biodiversity conservation and livelihoods.” [SPM pg.28]
Effective policies will move forward while accounting for risk and uncertainty, rather than waiting for total certainty.
Some risks are low-probability but high impact, and need to be taken into account. Assessment of “average” impacts may underestimate the benefits of mitigation.
“Climate policy may be informed by a consideration of a diverse array of risks and uncertainties, some of which are difficult to measure, notably events that are of low probability but which would have a significant impact if they occur. Since AR4, the scientific literature has examined risks related to climate change, adaptation and mitigation strategies. Accurately estimating the benefits of mitigation takes into account the full range of possible impacts of climate change, including those with high consequences but a low probability of occurrence. The benefits of mitigation may otherwise be underestimated (high confidence).” [SPM pg.5]
“Individuals and organizations differ in their degree of risk aversion and the relative importance placed on near-term versus long-term ramifications of specific actions [2.4]. With the help of formal methods, policy design can be improved by taking into account risks and uncertainties in natural, socio-economic, and technological systems as well as decision processes, perceptions, values and wealth [2.5].” [SPM pg.5]
To meet emissions targets, scenarios show a $30 billion/year decline in fossil fuel investment, a $147 billion/year increase in low-carbon energy investment, and a $100 billion/year increase in energy efficiency investments. Investments in long-lived fossil fuel infrastructure will be difficult and costly to change. These findings build on the AR4, which came to a similar conclusion that large shifts in investment were needed although the net increase in investments would be smaller.
“Substantial reductions in emissions would require large changes in investment patterns. Mitigation scenarios in which policies stabilize atmospheric concentrations (without overshoot) in the range from 430 to 530 ppm CO2eq by 2100 lead to substantial shifts in annual investment flows during the period 2010-2029 compared to baseline scenarios.” [SPM pg.29]
Keeping emissions to safe levels will require “a tripling to nearly a quadrupling of the share of zero- and low-carbon energy supply from renewables, nuclear energy and fossil energy with CCOS by the year 2050 relative to 2010 [about 17%).” [SPM pg.15]
“Infrastructure developments and long-lived products that lock societies into GHG intensive emissions pathways may be difficult or very costly to change, reinforcing the importance of early action for ambitious mitigation (robust evidence, high agreement). This lock-in risk is compounded by the lifetime of the infrastructure, by the difference in emissions associated with alternatives, and the magnitude of the investment cost.” [SPM pg.20]
“Over the next two decades (2010 to 2029) annual investment in conventional fossil fuel technologies associated with the electricity supply sector is projected to decline by about USD 30 (2-166) billion (median: -20% compared to 2010) while annual investment in low carbon energy supply (i.e., renewables, nuclear and electricity generation with carbon capture and storage) is projected to rise by about USD 147 (31-360) billion (median: +100% compared to 2010) (limited evidence, medium agreement). In addition, annual incremental energy efficiency investments in transport, buildings and industry is projected to increase by about USD 336 (1-641) billion (limited evidence, medium agreement), frequently involving modernization of existing equipment. For comparison, global total annual investment in the energy system is presently about USD 1200 billion.” [SPM pg.29]
“Future energy infrastructure investment decisions, expected to total over 20 trillion US$20 between now and 2030, will have long term impacts on GHG emissions, because of the long life-times of energy plants and other infrastructure capital stock. The widespread diffusion of low-carbon technologies may take many decades, even if early investments in these technologies are made attractive. Initial estimates show that returning global energy-related CO2 emissions to 2005 levels by 2030 would require a large shift in the pattern of investment, although the net additional investment required ranges from negligible to 5-10%.” [SPM pg.13]