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For many, it was a moment of unity and hope. Representatives from nations around the world had gathered in Paris in December 2015 and agreed to take drastic measures to curb climate change.听

But lurking behind the Paris Agreement was the understanding that those ambitious plans would not actually achieve the agreed-upon international goal of limiting global warming to an increase of 1.5 degrees Celsius, says Alden Meyer, a senior associate with E3G, a European climate change think tank.

There was an 鈥渁mbition gap,鈥� he says, so the parties agreed to reconvene and update their goals every five years.听

On Saturday, the Paris Agreement鈥檚 fifth anniversary, participating global leaders gathered once again 鈥� this time virtually 鈥� to reaffirm their commitment to the accord and to put forth even bolder plans.

鈥淚t鈥檚 all about putting some more meat on the bone,鈥� says Rachel Cleetus, policy director with the climate and energy program at the Union of Concerned Scientists. It鈥檚 about 鈥渢he countries not just making aspirational pledges, but backing them up with credible, ambitious policy plans domestically to make sure that they can meet their international commitments.鈥�

鈥淎n important step forward鈥�

The Climate Ambition Summit 2020 indeed delivered more tangible plans to reach lofty goals from many international leaders. But, experts say, there is still work to be done.

鈥淭he summit has now sent strong signals that more countries and more businesses are ready to take the bold climate action on which our future security and prosperity depend,鈥� United Nations Secretary-General Ant贸nio Guterres said on Saturday. 鈥淭oday was an important step forward, but it鈥檚 not yet enough. Let鈥檚 not forget that we are still on track to an increase of temperature of 3 degrees at least in the end of the century, which would be catastrophic.鈥�

For Saturday鈥檚 summit, 71 countries submitted more ambitious national climate plans. Many included significantly more aggressive goals than were outlined in Paris, and 45 focused on benchmarks for 2030.听The European Union, for example,听has now set higher goals than the ones it agreed to in Paris, promising听to reduce net carbon emissions 55%听from 1990 levels by 2030, a target that Mr. Meyer calls 鈥渁 significant step up in ambition.鈥�

Several countries pledged to go net-zero by the middle of the century 鈥� or sooner, in many cases. Perhaps most notably, Group of 20 countries like China, Japan, South Korea, the EU, and Argentina have joined that group.

When science meets politics

Since the Paris accord, some countries have taken strides in reducing their emissions. India, for example, has gone from being a growing source of global emissions to being on track to meet international plans to limit warming to 2 degrees Celsius, according to the Climate Action Tracker. The countries that have made notable progress tend to be smaller ones with minimal carbon emissions to start with.

But global carbon emissions have continued to rise, and most of the progress made in the last half a decade is hard to measure.听

There has been a move away from coal in much of the West, coinciding with falling costs of renewables, says Dr. Cleetus, making those energy sources competitive with fossil fuels in many parts of the world.听

鈥淲e know how to do 80% of emissions reduction,鈥� says Katharine Mach, associate professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science and a lead author for the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report and the U.S. Fourth National Climate Assessment.

鈥淭echnologically we know how to do it, economically, it鈥檚 the right choice, the lower cost option,鈥� in more and more places, she says. 鈥淭hat we can get most of the way there, to the super aggressive targets, based on the technologies that exist now, for me, is always very hopeful.鈥�

But this knowledge, she says, isn鈥檛 necessarily easy to translate into climate action. There鈥檚 politics to contend with.听

鈥淐limate models usually have been just physics of how the science works, or in the solutions model space, it鈥檚 just straight economics,鈥� she says. 鈥淚n between the two is how societies actually work.鈥�

A fickle friend

To Michael Mann, professor of atmospheric science at Pennsylvania State University and a lead author on the Observed Climate Variability and Change chapter of the IPCC Third Scientific Assessment Report, 鈥渁mbition is a close cousin of optimism and hope.鈥�

鈥淓very bit of additional warming does damage, so we can never truly be too ambitious,鈥� he writes in an email. 鈥淏ut we must also recognize the political constraints we鈥檙e operating under and work to achieve progress that is possible under those constraints.鈥�

A big concern for Paris Agreement participants and climate change activists is the absence of action from some of the biggest emitters, like Australia and the United States. The U.S. emerged as a leader in climate pledges five years ago, but with the nation pulling out of the agreement under President Donald Trump, that changed sharply.听

Now, President-elect Joe Biden, who made climate change a central part of his campaign, has said he plans to rejoin the Paris Agreement. But much of the world is waiting to see what can actually be achieved when he takes office in January.

鈥淭he world is painfully aware of the yo-yo nature of American politics鈥� and the partisan split, says Mr. Meyer. 鈥淭he question people overseas have is, what can the U.S. deliver, given the political tensions?鈥�