Having agreed to limit global warming, the COP must now figure out how to measure progress toward the goals set in Paris. Without double-checking this progress, the climate goals won't be reached.
Two years after the world committed to climate action in Paris, negotiators are still trying to figure out the nuts and bolts of the global deal to limit global warming to less than 2 degrees Celsius.
A big part of that is developing a plan to monitor and verify the pledges made by nearly 200 countries to reduce greenhouse gas emissions. During the two-week long COP23 talks in Bonn, negotiators are focusing, in part, on developing a system to make those measurements.
Writing that rule book is no easy task, says University of Edinburgh climate scientist Paul Palmer, part of an international team that assesses heat-trapping gases in the atmosphere.
"We need to measure carefully. We'll be looking for small, gradual reductions of large numbers, so we need to make sure we get the numbers right," Palmer told DW.
Why is a verifiyable rulebook important to reach the Paris goals?
Because the measures countries have committed to under the Paris Agreement are voluntary, checking whether parties are actually making progress will be key, says Andrew Light, a former US climate negotiator under the Obama administration, now a senior fellow for climate policy at the World Resources Institute.
"Transparency is the main way we're going to know whether or not we fulfill the promise of Paris," he told DW. "The only way to know that we're making progress is our ability to understand what other parties are doing."
For now, the world relies on self-reporting and peer review, which involves close scrutiny of the way each country carries out its greenhouse gas inventories.
How do countries report their progress?
There are many sources of greenhouse gases, and each country is responsible for tracking its own emissions and showing demonstrating progress on cuts.
Essentially, countries quantify how much coal, oil and gas they use and rely on a standard formula to calculate greenhouse gas emissions. The equations also include numbers on farming and forestry practices to account for greenhouse gases from livestock, fertilizer use and changes in tree cover.
By when do the rules need to be completed?
The rule book needs to standardize countries' accounting procedures, and it needs to be done by 2018, according to the Paris Agreement's timeline. For that to happen, delegates in Bonn have to start agreeing on carbon-counting details.
Light said one of the main hurdles to transparency is the "bifurcated" system of accounting adopted at the 2010 climate conference in Cancun, "with different rules for developing and developed countries … and more scrutiny and oversight for developed countries."
How involved will the US be with drawing up the rulebook?
In order for Paris to work, everyone eventually has to play by the same rule book, Light says. And despite President Donald Trump's plans to pull the United States out of the Paris Agreement, the US still co-chairs the committee on transparency - with China - and still has some of the most advanced ideas on how to make that happen, he adds.
"It's a very interesting place we're in, with Trump's announcement to withdraw from Paris," Light said. "The US has the most progressive views on moving toward a uniform [accountability] system, but it's not clear how that will play out at the Bonn talks."
Quantifying greenhouse gas emissions is complicated - not only fossil fuel emissions have to be counted, but also deforestation and agriculture
Observers shouldn't expect any groundbreaking announcements on this at COP23, says Glen Peters, research director at the Center for International Climate Research in Oslo.
"It will happen slowly, over five- and 10-year increments," he said, acknowledging that the split accounting system can't work in the long run.
What are the rules for developing countries?
"Developing countries have to periodically submit reports, but don't have to provide detailed emissions estimates. So we have quite little official understanding of what their emissions might be. That makes it really difficult," Peters said.
Even though there are independent estimates, for example from the International Energy Agency, they don't always use the same yardstick, so there are still big uncertainties about emissions in countries like China, Brazil and India, he adds.
In particular, he called on China to step up monitoring and verification. Smaller, less developed countries might be able to make a reasonable claim that they lack resources to generate detailed reports, but China has the skills and capacities.
"China is trying to show it's a leader. So, open your books - that will put pressure on other developing countries like India and Brazil to do the same," climate researcher Peters said, adding that the Global Carbon Project will be releasing a report on November 13 with projections for how long it might take before meaningful changes in emissions can be detected.
How will CO2 monitoring work in the future?
Technology, especially satellites, will play a bigger role in monitoring and verifying greenhouse gas emissions, and in analyzing the global carbon cycle in general.
The Orbiting Carbon Observatory will gather data from parts of the globe on-the-ground measurements cannot reach
That's important because fossil fuels are not the only source of greenhouse gases. The last three years are the perfect example for that. Even though annual emissions from fossil fuel sources remained constant from 2014 to 2016, the overall concentration of CO2 in the atmosphere increased dramatically faster than in previous years.
A new NASA satellite mission, the Orbiting Carbon Observatory 2, traced that increase to changes in tropical forests in the Amazon region, as well as Africa and Indonesia.
Satellites, Edinburgh climate scientist Palmer says, will gather data from countries where financial or political factors get in the way of on-the-ground measurements — as well as areas that just aren't physically accessible.
"They'll fly over major forested ecosystems where sustaining a measurement network is very difficult because of the technology and the necessary human support, and over the major oceans where they can measure continental outflow," he explained.
"That will help untangle emissions from oil, gas and coal burning from natural processes and land-use changes."