Budgeting for climate change

As the world's leaders are negotiating climate change mitigation in Paris, a strong El Niño brings the warmest year on record. After a decade and a half of slow warming and slow policy progress, 2015 may bring an acceleration of both.

Policymakers seem ready to take new steps to tackle climate change. Research must draw on lessons from the past to find productive pathways for the future.

Rapid growth in global CO₂ emissions from fossil fuels and industry ceased in the past two years, despite continued economic growth. Decreased coal use in China was largely responsible, coupled with slower global growth in petroleum and faster growth in renewables.

In the absence of an enforceable set of commitments to reduce greenhouse gas emissions, concerned citizens may want to supplement international agreements on climate change. We suggest that litigation could have an important role to play.

Some climate change impacts rise fast with little warming, and then taper off. To avoid diminishing incentives to reduce emissions and inadvertently slipping into a lower-welfare world, mitigation policy needs to be ambitious early on.

Multi-actor integrated assessment models based on well-being concepts beyond GDP could support policymakers by highlighting the interrelation of climate change mitigation and other important societal problems.

Delivery of palatable 2 °C mitigation scenarios depends on speculative negative emissions or changing the past. Scientists must make their assumptions transparent and defensible, however politically uncomfortable the conclusions.

Many governments agreed to limit global mean temperature change to below 2 °C, yet this level has not been assessed scientifically. A synthesis of the literature suggests that temperature is the best available target quantity, but a safe level is uncertain.

Greenhouse gas emissions can be allocated to individual countries in various ways depending on where in the supply chain the emissions originated; achieving an effective and just climate policy may require multiple accounting systems.

Scenario analyses suggest that negative emissions technologies (NETs) are necessary to limit dangerous warming. Here the authors assess the biophysical limits to, and economic costs of, the widespread application of NETs.

Humans have altered their environment ever since they first appeared. Updates on three frameworks of thinking about the scale of twenty-first-century human influence on the Earth are invigorating the global change debate.

Bioenergy with carbon capture and storage could be used to remove carbon dioxide from the atmosphere. However, its credibility as a climate change mitigation option is unproven and its widespread deployment in climate stabilization scenarios might become a dangerous distraction.

The emerging scientific focus on cumulative carbon emissions may make climate negotiations harder. But, it serves to clarify the scale and scope of climate mitigation needed to meet potential temperature targets.

For the first time since the failed 2009 Copenhagen Climate Change Conference, momentum is building towards a new climate agreement. But expectations must be kept in check, and making expert advice more useful to the process will require engaging the social sciences more fully.

Future cumulative CO₂ emissions consistent with a given warming limit are a finite common global resource that countries need to share — a carbon quota. Strategies to share a quota consistent with a 2 °C warming limit range from keeping the present distribution to reaching an equal per-capita distribution of cumulative emissions. This Perspective shows that a blend of these endpoints is the most viable solution.

Experts using integrated assessment models to analyse the effects of climate change policy, have recently engaged in model inter-comparison projects (MIPs) to generate conclusions robust to different models' specifications. This Review synthesises results from the most comprehensive MIP that focuses on the different possible outcomes of post-2020 climate negotiations, recently announced pledges and their relation to the 2°C target.

In order to limit climate warming, CO₂ emissions must remain below fixed quota. An evaluation of past emissions suggests that at 2014 emissions rates, the total quota will probably be exhausted within the next 30 years.

The relative uncertainty of anthropogenic climate forcing has decreased in the past decade. A statistical model suggests that by 2030 this uncertainty will be halved, as CO₂ increasingly dominates over other human-made climate influences.

Transient global warming is nearly proportional to cumulative carbon emissions. A theoretically derived equation shows that this relationship stems from the partially opposing climate effects of oceanic uptake of heat and carbon.

Breaking away from the utopian assumption that the international community will agree on a single emissions allocation scheme, this study assesses approaches to setting country-level mitigation targets in line with the 2 °C goal.