Synthesis Report

IPCC AR5 Synthesis Report Overview

The final installment of the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report (AR5) is due to be approved and released next week. This document, the Synthesis Report (SYR), will combine the findings of the first three Working Group Reports to deliver clear and comprehensive takeaways.

Together, the Working Group Reports represent the most comprehensive review of climate science ever attempted. They conclusively reaffirmed a set of basic findings:

  • Humans are warming the climate, with the best estimate being that humans are responsible for all warming since 1951.
  • Warming is already proving costly and detrimental, and climate change is already increasing the frequency and severity of many types of extreme weather.
  • In the future, unhindered greenhouse gas pollution would cause impacts that would threaten food security, international peace, economic growth, and more.
  • Redirecting investments towards clean energy development – and not to fossil fuel infrastructure - is critical to meeting temperature targets and avoiding these harmful impacts.

By necessity and design, the SYR cannot include all of the findings of all three reports, but it has been carefully crafted by hundreds of scientists to reflect the balance of the evidence. As such, it will provide an invaluable tool to decision-makers. In broad strokes it lays out a choice between two paths depending on our energy choices today: a 4ºC warmer future powered by fossil fuels and severely disrupted by climate impacts, or a future based on clean energy where economic growth can thrive in a stable environment.

The following backgrounder will serve to provide an overview of the most noteworthy instances where the SYR can integrate findings from the working groups to provide useful clarity. Following that is a review of the top individual findings of the previous three working groups.


Synthesis Report: Overarching Conclusions

Noteworthy expected findings

  • Business as usual will result in extremely harmful consequences (more)
  • The energy sector represents one of our most urgent decision points between a carbon-polluted path or a clean path (more)
  • Adaptation alone will not adequately deal with the impacts of climate change (more)
  • Tipping points introduce dangerous negative uncertainties into climate projections (more)
  • Decision makers should account for possible catastrophic outcomes (more)
  • Delaying mitigation will substantially increase risks, costs, and challenges (more)
  • Available evidence indicates that the cost of inaction is far higher than the cost of mitigation (more)


Business as usual will result in extremely harmful consequences.

If greenhouse gas pollution is not checked, warming is projected to exceed 4ºC by 2100. This scenario would be accompanied by species extinctions, food insecurity, economic poverty, and other severe disruptions. Migrations from regions rendered impractical for normal life could be a reality. Coral reefs and fisheries would be decimated. The IPCC remains reserved about using subjective words like “dangerous” or “catastrophic” to describe these consequences, but a reasonable person might find them applicable.


The energy sector represents one of our most urgent decision points between a carbon-polluted path or a clean path.

WGIII makes the point that long-lived fossil fuel infrastructure could lock us in to a high-emissions growth pathway for decades to come. This would eventually yield a 4ºC warmer future that would catastrophically strain society’s resources. Conversely, investments in clean energy and smart development could yield a future of strong economic growth, while keeping emissions down. The global annual consumption cost of pursuing low-carbon development (0.06% per year according to WGIII) represents little more than a rounding error, presenting a stark contrast to the uncertain yet massive costs of a 4ºC warming scenario.

For a more targeted, non-IPCC dive into these numbers, see the recently released New Climate Economy Report, which found that $90 trillion of infrastructure investments will be required globally in the next 15 years, and that directing this capital towards low-carbon investments would increase upfront costs by only about 5%, while delivering a host of co-benefits to health and welfare.


Adaptation alone will not adequately deal with the impacts of climate change.

Adaptation is costly, and research already shows a disparity between adaptation needs and available funds. Even with full commitment of funds, the impacts of a high-emissions scenario would exceed the scope of many types of adaptation. In fact, adaptation will be necessary alongside mitigation, due to the ongoing effects of warming thus far. Food production represents an especially critical area where adaptation may alleviate the effects of warming up to a point, but will rapidly prove inadequate as we approach a 4ºC warmer world.


Tipping points introduce dangerous negative uncertainties into climate projections. 

Tipping points are abrupt and irreversible changes, whose risk increases with added warming. New evidence since the Working Group (WG) Reports were finalized indicates the tipping point for the collapse of the Antarctic ice sheet may already be behind us, which would mean existing projections of sea level rise could be too low. Other tipping points could be passed in releasing critical amounts of stored methane and carbon from melting permafrost, and ecosystems could also face collapses if they take critical hits in terms of biodiversity loss. Considerable uncertainty over when and how tipping points might occur only underscores the need for action now to avoid setting off these impacts.


Decision makers should account for possible catastrophic outcomes.

People have natural biases towards considering the short term while forgetting impacts that are farther off, as well failing to consider near-term impacts that may be uncertain, yet high-consequence. It may not be possible to objectively determine what mitigation methods are the best, but decision makers would benefit from using analytical techniques drawn from science and economics. This may seem to be simple common sense, but its inclusion in the report would serve as a hidden rebuke to decision makers’ shortsightedness thus far.


Delaying mitigation will substantially increase risks, costs, and challenges.

It will require substantial reliance on technologies unproven at scale including methods to remove carbon dioxide from the atmosphere and sequester it. Overall cost of mitigation measures would be significantly higher (about 40% according to WGIII) in scenarios where mitigation is delayed, compared to scenarios where it is started immediately. Delaying action also results in reduced chances of successfully meeting temperature targets.


Available evidence indicates that the cost of inaction is far higher than the cost of mitigation.

Existing science doesn’t provide the hard numbers necessary to do an apples-to-apples cost comparison (due to large uncertainties on the costs of tipping points, technology development, and more). For example, WGII provides aggregate costs of warming only up to 2ºC. This does not imply the absence of severe costs to climate inaction. Warming this century could exceed 4ºC, and existing evidence indicates costs could skyrocket.


Working Group I (WGI): The Physical Science

Noteworthy findings (for more, see CN overview)

Climate change is primarily caused by humans. WGI found with 95% certainty that humans cave caused most of the observed warming since 1951, and made the “best estimate” that humans have caused all of it.

Many indicators of climate change are accelerating. Ice sheet melt, glacial melt, and sea level rise have all accelerated faster than previously predicted.

Climate change is impacting extreme weather. Increases in the frequency and intensity of some types of extreme weather have been detected today, and other types are projected to increase in the future.


Working Group II (WGII): Impacts and Vulnerabilities

Noteworthy findings (for more, see CN overview)

People everywhere are vulnerable to extreme climate and weather events. Adaptation and preparation remains low, which could result in severe consequences as impacts rise.

Climate change is already harming agricultural yields, and this will worsen. Recent rapid increases in food prices show that markets are sensitive to climate variability. Potential benefits to warming in some localized regions will not be sufficient to offset the negative impacts.

Further delay will cause temperature targets to be exceeded. Temperature targets such as the 2ºC of warming agreed upon by world leaders will soon be out of reach if no further action is taken.


Working Group III (WGIII): Mitigation of Climate Change

Noteworthy findings (for more, see CN overview)

The energy sector will need to be a leader in the transition to a low-carbon economy. Renewable energy will need to triple or quadruple by 2050 to meet temperature targets. Meanwhile, carbon dioxide from fossil fuels and industry has accounted for about ¾ of greenhouse gas pollution since 1970.

Restricting warming to 2ºC is practical and affordable. The cost of renewable energy is falling and deforestation rates have declined. Ramping up investments in climate mitigation would have only a small impact on global economic growth.

Investments in long-lived fossil fuel infrastructure could lock us into a harmful emissions pathway. To meet 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