The window for limiting climate change to 2C at a feasible economic cost is rapidly closing, but science is the basis for finding the right solutions. That’s the conclusion of the Our Common Future under Climate Change conference in Paris this week.

Keeping global temperature rise to 2C or less will mean greenhouse gas emissions must be zero or even negative by the end of the 21st century, the conference’s scientific committee says. That will require “bold action” starting now – delaying deep emissions cuts or not pursuing clean-energy technologies will only make solutions more difficult and more costly later down the line. “2015 is a critical year for progress,” it warns.

Carbon Brief has been at the conference all week. Here’s our summary.

A packed schedule

This week has seen thousands of experts come together for the largest scientific gathering on climate change before COP21 in Paris in December.

An event such as this was always going to attract a huge range of interests and disciplines, and the breadth of the programme didn’t disappoint.

Climate change and its impacts were the focus on Tuesday when the conference opened, with talks ranging from measuring ice sheet loss from space to extreme events and how climate change is eroding the cultural heritage of Paris. Days two and three were more about weighing up the different options on the table for reducing emissions, with discussions on everything from limits to negative emissions to renewable energy in China. A highlight of the week on day two was a lunchtime debate about the Guardian’s Divestment campaign and the rights and wrongs of advocacy journalism. The week draws to a close today with a spotlight on what’s next for climate science and policy, and questions of collective action and global governance.

You can catch up on each day’s best bits with Carbon Brief’s daily highlights.

Statement of intent

To conclude the week, the conference’s scientific committee has distilled all of what’s being going on into some key conclusions, which lean heavily on the latest IPCC report’s findings.

Limiting warming to 2C is still possible at a reasonable cost, today’s statement says, but this requires reducing greenhouse gas emissions by 40-70% below current levels by 2050.

How much we mitigate over the next few decades will determine how much warming we are committed to in the long-term, it says.

So how do we tackle the problem? The committee recommends a range of options. These include putting a price on carbon and phasing out subsidies on fossil fuel-based energy to create a level playing field among all types of energy generation.

But the impacts of climate change are already “widespread and consequential”, the committee points out. Impacts have affected every continent, though people are most vulnerable where poverty, inequality and lack of government leadership hinder adaptation, it says.

There is some good news in the statement, too:

“Investments in climate-change adaptation and mitigation can provide a wide range of co- benefits that enhance protection from current climate variability, decrease damages from air and water pollution, and advance sustainable development.”

Creating a common future with “sustainable, robust economies, equitable societies, and vibrant communities” is intertwined with tackling climate change. And science is the foundation for making the decisions necessary to get there, the committee concludes.

Reaction

With such a packed programme and its billing as the biggest scientific meeting ahead of the COP, what do climate scientists think has been achieved this week?

Big gatherings like this drive the science forwards and connect the dots between different disciplines, Dr Richard Betts, head of climate impacts at the Met Office, tells Carbon Brief.

“[What we’re all doing here is] digging into the meaning of the scenarios that we’ve developed, not only what they mean for climate change, but also other things like land use, biodiversity, aerosol emissions, air quality â?¦ We’re trying to get beyond just ‘is the world warming and what does that mean?’ [to a] more holistic picture. So that if we are making long term planetary strategic decisions, [we know] what they mean for everything.”

The very existence of the conference sends a signal to negotiators at the Paris COP in December, says Stanford University’s Prof Chris Field, chair of the scientific committee for this conference and co-chair of the IPCC’s working group three report. Field tells Carbon Brief:

“In many ways, this conference and the 2000-and-some scientists are really gathered to express, more than anything else, their willingness to be part of the process and their commitment to making sure that their knowledge is available as a foundation going into the Paris COP.”

What would success in Paris would look like? Field tells Carbon Brief:

“Coming from the scientific community, my idea of success is that the negotiators use the foundation from science to produce an ambitious, meaningful agreement.”

Prof Ottmar Edenhofer, chief economist at the Potsdam Institute for Climate Research (PIK) and co-chair of the IPCC’s working group three report on mitigation, says it’s important to realise that Paris is not the end game. He tells Carbon Brief:

“We should not see this conference as the scientific input for the [UNFCCC] negotiations. The science is quite clear now. For that, there is no need for such a conference. The purpose of the conference was to generate new ideas after the Paris COP. We should not just focus on the Paris COP. There is a day after Paris, too. Climate policy is not a sprint, it’s a marathon.”

Prof Kevin Anderson, professor of energy and climate change at the University of Manchester, agrees the sightlines should extend beyond Paris, but tells Carbon Brief there’s still everything to play for. He says:

“We need to try to hold to 2C. If we don’t hold to that then we need to hold to the lowest temperature we possibly can.”

An economist’s view

On the final day, the Nobel laureate Joseph Stiglitz had top billing in the keynote speeches. From his viewpoint as an economist, he discussed why tackling climate change in the aftermath of a global financial crisis is not necessarily bad timing:

“We have an opportunity to strengthen the global economy at the same time as we address the problem of climate change.”

Creating a green economy is not only consistent with economic growth, it can promote economic growth, argued Stiglitz. But how do we get international cooperation on climate change in the first place?

Existing approaches aren’t working, he said. Rather than trying to agree a cap-and-trade system, or relying on voluntary emissions reduction, a global carbon price is the way forward, he argued.

Just a moderate carbon price would create enough revenue to reduce other taxes, keeping the overall economic burden low or even negative for some countries, said Stiglitz:

“This reflects the basic economic principle: that it’s better to tax bad things than good things.”

For those countries that didn’t join the “coalition of the willing” of a global carbon price, they could be penalised through cross-border taxes, which – he added – would be legal according to the World Trade Organisation.

And as the costs of climate change are likely to be especially great for developing countries, a portion of revenues from the carbon price could be allocated to them – but only if they adhered to the carbon price as well, he suggested.

Many of the presentations from the conference, including the one given this morning by Stiglitz, have now been placed online. A Youtube channel, featuring recordings from many of the sessions, has also been created.

Main image: Our Common Future Under Climate Change conference, 2015.