Continuing to pump greenhouse gases into the atmosphere will trigger "severe, pervasive and irreversible impacts for people, species and 27 ecosystems," concludes a landmark draft U.N. science report expected to be approved this week.

Adapting to climate change, according to a final draft obtained by ClimateWire, can reduce some risks. But, it argues, "there are limits to its effectiveness, particularly if greenhouse gas emissions are not reduced."

In order to keep global temperatures from rising above 2 degrees Celsius beyond preindustrial levels—the point at which experts predict the planet will experience the irreversible effect of climate change—net global emissions must plummet 40 to 70 percent by 2050, hitting zero by the end of the century. And yet, the report will argue, there is still time to act.

"Risks from mitigation can be substantial, but they do not involve the same possibility of severe, widespread, and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation action," the draft notes.

The sythesis report by the Intergovernmental Panel on Climate Change (IPCC) pulls together three sweeping scientific works that the United Nations' top climate science body has released over the past 13 months. They include documents detailing the latest climate science, the impacts to ecosystems and people, and options for reducing emissions. Scientists and delegates from more than 100 countries are meeting behind closed doors in Copenhagen, Denmark, this week to complete it as well as an even shorter "summary for policymakers" that will be released Sunday.

It comes as governments prepare to meet in Lima, Peru, for annual U.N. negotiations toward an international climate agreement. The deal is expected to be signed in Paris at the end of 2015, and several officials noted that the bite-sized summary could help put global warming on the radar screen of world leaders over the coming months.

"Policymakers like short, up-to-date documents that present a clear, balanced view of the key issues around the problem on their table," said IPCC Vice-Chairman Jean-Pascal van Ypersele. "It will also, most probably, contain a catchy series of headline statements, which will provide easy to remember elements of context for the impending climate negotiations."

The headline statements will make the report more digestible "in the same way that a cake is much more palatable than the dry ingredients it is made from," he said. "Climate negotiations need to be informed by the best assessment of scientific information. That is what the IPCC Synthesis Report will provide."

Challenges from the developing world
Yet the summary is more than just a wrap-up of the science; it is a road map for negotiations, which means that each statement could hold nations politically accountable in the future—so nations consider wording with great care.

Several points of disagreement exist between developing and developed nations, said Sanjay Vashist, director of the Climate Action Network.

The draft report states that if nations do not cut emissions further, warming could exceed 4 C. This would result in "severe, widespread, and irreversible" impacts. It would lead to "substantial species extinction, global and regional food insecurity, consequential constraints on common human activities, and limited potential for adaptation in some cases," the report states.

Several observers called for greater clarity in wording in the report, according to observers who said they expect that section to be a subject of debate.

The report does not state that the impacts of climate change would disproportionately affect the developing world, Vashist said.

"The reason it is so far being indirectly referred to is because developed countries would not like that losses and damage clearly coming out in IPCC," he said, "because then it will be politically a burden on them, and they will be under pressure to deliver."

India is challenging the "carbon budget," which refers to the hypothetical limit to fossil fuel combustion the world must abide by to limit warming. The draft states that the world has a maximum of 2,900 gigatons of CO2 to emit, of which two-thirds has already been used up.

"That curtails the development agenda of developing countries, so India would like to make sure that if it is articulated in the SPM, then it should be in a manner that more is available for developing countries and mitigation ambition is raised for developed countries," Vashist said.

The United States, others noted, also has concerns about the carbon budget. Indeed, the debates over the wording describing where responsibilities should lie for tackling climate change are expected to be the most sensitive.

"I think that's going to be an important one to see where that lands in the synthesis report," said Leo Hickman, chief climate change adviser to WWF-UK. "It spells out that we have this finite amount of carbon left to burn. The numbers are there, and it's pretty clear what road we're on and what road we need to take."

The draft also mentions that decarbonization by reducing the carbon intensity of electricity generation is an important strategy to cut emissions. India would have a problem with that, since coal provides up to 50 percent of its primary energy mix, Vashist said.

"For India, it is an issue of curtailing development and leaving millions without any basic amenities," he said. "This is a red line for India."

Act fast, or feel the heat
Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment, said: "It will come down to the discussions over who does what. I don't think there's going to be many people arguing about the risks. That's pretty much agreed now. But this ties into the negotiations process, and there are some countries that still, unfortunately, regard the negotiations as a zero-sum game where some countries have to lose for other countries to win."

The IPCC treads lightly on the topic of extreme weather events. It states that cyclones, floods, droughts and other extreme weather events in recent times have exposed the vulnerabilities of current infrastructure, but does not go into further detail on storms and precipitation anomalies. Instead, it focuses most of its attention on heat waves, which it states have become more frequent in Europe, Asia and Australia.

If CO2 emissions are not controlled, heat waves will very likely occur more often and be of longer duration, the report states. In the business-as-usual state, Arctic sea ice would disappear in September before 2050, glaciers would continue to retreat, oceans would acidify and global sea levels would rise.

"Climate change will create new risks for natural and human systems and amplify existing risks in countries at all levels of development," the report states.

A key message of the draft is that acting now and acting fast are needed to limiting warming to 2 C above preindustrial levels by 2100. Delayed action would be more expensive and challenging, the draft suggests.

Still, the report strikes a note of hope that the 2-degree target, which some observers have questioned in recent months, is achievable (ClimateWire, Oct. 3).

To limit warming to below 2 C, the world would have to constrain carbon dioxide levels in the atmosphere to 450 parts per million (ppm) by 2100, according to the report. The concentration is currently hovering at 400 ppm and growing quickly.

To curb CO2 emissions, the world would have to change its energy system and alter land-use patterns by midcentury, the draft states. And nations would have to phase in mitigation strategies by 2030 to avoid even bigger challenges in the future, the report states.

"Implementing such reductions poses substantial technological, economic, social, and institutional challenges, which increase with delays in additional mitigation and technology constraints," the report states.

The IPCC states that even a 1.5 C target would be plausible if nations adopt mitigation and adaptation strategies before 2030.

Kaisa Kosonen, a climate and energy campaigner at Greenpeace Nordic, said the low targets would be possible if the world were to switch to renewable energy and adopt other measures.

"It is really up to us to make those decisions and get there," she said. "We are no longer in the business of just controlling emissions or managing them; we need to phase out fossil fuel emissions altogether, and we need to start very soon. The more we delay, the harder it will be to get there."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500