“Historic.” “A game-changer.” “Landmark commitments.” “A very big deal.” The superlatives describing the US-China climate deal announced on Tuesday have come thick and fast.

Chinese leader Xi Jinping announced for the first time that it will cap emissions, setting 2030 as an explicit target for when the country’s greenhouse-gas emissions are to peak. Until now, the People’s Republic had only ever said it would reduce its carbon intensity—the amount of CO2 emitted per unit of production. In addition, Beijing is to ramp up its production of energy for zero-emission sources to 20 percent by the same date.

As part of an agreement secretly negotiated over the past nine months, President Barack Obama hiked his country’s CO2 emissions reduction target to 26-28 percent below 2005 levels by 2025, up from 17 percent by 2020.

These are the headline figures. The deal also includes a commitment by the two countries to cooperate on clean energy research and development, including a carbon-capture and storage (CCS) pilot project. And Washington and Beijing will extend their existing cooperation on a phase-out of hydrofluorocarbons, the “super greenhouse gases” with a warming effect almost 4000 times more potent than CO2.

The two nations together represent roughly 40 percent of global emissions, but both sides have until now have been unwilling to commit to deeper cuts. As a developing nation, the Chinese have argued that they are not responsible for the historic emissions that have resulted in global warming, while the Americans have been reluctant to pledge greater climate action until China moved, arguing that China has developed significantly since the world first became aware of the problem.

Diplomats, government leaders and environmental NGOs have on the whole cheered an end to the stalemate, although some have been more cautious, while others, including some climate scientists, have been more critical.

Where does the truth lie? Is this a breakthrough or is it little different to business as usual? What significance does the announcement have for global negotiations? And how likely are the two sides to succeed in meeting their pledges?

Glen Peters, a researcher with Norway’s Centre for International Climate and Environmental Research and the Global Carbon Project, which tracks emissions and mitigation pledges, told Road to Paris that beyond the hype, the Chinese goal of peaking emissions by 2030 remains “very vague.”

“A 2030 peak could mean emissions 30-50% higher than today,” he says. “To keep global temperatures below 2C, China would need to take quite significant mitigation.”

Some analyses already suggested that China’s emissions are likely to peak in 2030 anyway. In order to meet the internationally agreed limit of 2 C of warming of average global surface temperatures, Peters and his colleagues estimate that they would have to peak instead at some point between 2015 and 2020, and then decline roughly 8-10 percent per year afterward.

No matter which metric is chosen to share out the planet’s remaining carbon budget—whether allotted on a per capita basis, a key demand of the developing world, or on the basis of current emissions, the preference of the developed world—the country would still have to engage in gargantuan emissions reductions, Peters says. This is due to the sheer volume of China’s emissions, a share that is increasing.

“People tend to think of the China that existed in the 1990s. China is lucky in the sense that many don’t realise how much it has changed over time. Whenever you mentioned Chinese emissions, there is always a comment about per capita emissions with the impression they are still quite low,” he continues. “But Chinese emissions have grown immensely since around 2000.”

In 2002, Chinese emissions represented roughly 14 percent of the global total. At this time, the International Energy Agency believed that even in 2030, the country’s emissions would still be far below those of the US.

Since then, China’s emissions have climbed 8 percent annually on average, overtaking the US in 2006. As of 2013, China now emits 28% of global emissions, has greater emissions than the US and EU combined, and has per-capita emissions 45 percent above the global average, emitting more than the EU on a per capita basis. “These three factors are also growing. Chinese emissions are high, no matter what scale you put them on.”

Even taking into account historic emissions since the Industrial Revolution, China comes third behind the US and EU.

So if this deal represents a tacit concession of the leading role that China must play alongside the traditionally defined rich world, then this is a significant diplomatic switch from Beijing’s stance. Up to now, China, alongside climate allies such as India, Brazil, South Africa and the other 133 member states of the G-77, has argued that the internationally agreed principle of “common but differentiated responsibilities” meant that they had the right to continue to develop their economies using carbon-intensive methods.

“Another interesting aspect for China is that they themselves will face significant impacts [from climate change], and thus it is in their own interests to reduce emissions radically,” adds Peters. “I think China could play a more aggressive role in negotiations, and China could play a key role in forcing other countries to move.”

Nevertheless, even with the diplomatic shift, the question is whether the concession is enough. And on that front, Peters and others reckon that China’s pledges are not very different from business as usual.

“Overall, the Chinese and US emissions target represent a political step forward, but are broadly not consistent with a likely chance of keep temperatures below 2C. The targets themselves are not so different from the continuation of existing trends. The announcement represents one small step in a very long journey.”

What will make a difference is less its pledges, but whether the country can successfully wean itself off coal, and relatively rapidly. And on this front, the People’s Republic’s development of clean energy infrastructure is unprecedented. It’s just that its growth in coal-fired infrastructure is unprecedented too.

China has been for some time the largest consumer of coal in the world, with coal delivering roughly 70% of its energy. And once built, coal-power plants have a lifetime of operation of around 40 years, so China has already locked in more CO2 emissions than it has emitted up to now.

However, particulate from coal has resulted in sufficient smog levels for air pollution to become a major concern amongst its citizens, and international companies who consider such issues when deciding whether to set up shop. And so Beijing’s “war on pollution” has made the shift to clean energy sources, notably wind, solar, hydroelectric and nuclear, a domestic as well as international imperative. China is now the world’s largest producer of wind energy and second largest producer of solar energy.

Selling the deal, the White House has stressed the scale of mobilization that will be required for China to meet its new commitment to 20 percent zero-emission energy production: “It will require China to deploy an additional 800-1,000 gigawatts of nuclear, wind, solar and other zero-emission generation capacity by 2030—more than all the coal-fired power plants that exist in China today and close to total current electricity generation capacity in the United States.”

The government is also considering a carbon tax, and carbon trading schemes similar to Europe’s faltering system are currently being piloted in a handful of cities and provinces.

But of all its options, the Chinese leadership is placing most of its chips on a revolution in nuclear power. A total of 28 nuclear plants are under construction, atop the 20 currently in operation, giving China a nuclear sector growing faster than anywhere else in the world. The country hopes to boost its nuclear capacity to 88 gigawatts by the end of the decade, an increase of 80 percent, and 400-500 gigawatts by mid-century. For comparison, the Chinese electricity system alone currently produces 1,250 gigawatts of power.

China watchers predict that the government’s next five year plan, running 2016-2020, will prioritise clean energy and nuclear in particular. This means that alongside global UN talks, climate watchers will need to be keeping an eye on the fine print of the five-year-plan when the general guidelines are formed between December and February in the new year, and as the plan is developed over the course of 2015, to see how aggressive the build-out will be when finalized.

China’s emissions are growing 8 percent a year, but they have to be reducing their emissions by at least 8 percent a year. Meanwhile, the most successful decarbonisation programmes in history, the transition to nuclear power by France, Sweden and Belgium in the 1970s and 1980s, with France now producing 78 percent of its electricity from this low-carbon source, enjoyed reductions in emissions of four percent a year over the course of roughly a decade.

So even to achieve what China promised this week would be a colossal undertaking. To achieve what is necessary though would be elephantine. Yet China is also the country that builds a new skyscraper every five days, and has in the last few years constructed some 30 airports, 25 subway systems, the three longest bridges, the largest hydroelectric dam, and thinks nothing of flattening 700 mountains to throw up a brand new metropolis for 500,000 residents from scratch.

But what about the other side of the deal? How does America’s pledge stack up?

While China’s pledge is inconsistent with a two-degree target, regardless of how the world’s remaining carbon budget is shared out, according to the analysis of Peters and his colleagues, the new American commitment increases the level of ambition from emissions cuts of around 1.2% per year under its previous pledge, up to around 2.3-2.8% per year. For comparison, this rate of emissions reduction just about beats that of Norway.

“The US pledge is roughly consistent with a 2C target, if it is assumed that emissions are distributed based on current emission levels,” Peters calculates. “If equity is considered, such as population based sharing of emissions, then the US target is inconsistent.”

And then there is the question as to whether the US will be able to meet its pledge. Obama has two years left in his mandate and Republicans last month captured the Senate from the Democrats. Republicans have consistently argued that the US cannot move on climate change until China acts, so this deal should have removed that hurdle.

And for all the conservative climate scepticism in the US, exit polls from the November congressional elections showed that 58 percent of voters—those conservative voters that delivered a Republican landside—are concerned about climate change.

Yet Republicans remain steadfastly opposed. “This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs,” was the reaction of Mitch McConnell, the party’s leader in the Senate.

The president does not need approval of Congress to act however. And, since the defeat of 2009’s American Clean Energy and Security Act, which would have introduced an EU-style emissions trading system, Obama has moved to use his regulatory power under the country’s Clean Air Act to achieve emissions reductions at new and existing power plants instead. Other mechanisms at his disposal include vehicle efficiency standards. But these policies are unlikely to achieve a 28 percent cut all by themselves without new clean-energy infrastructure.

There is an additional, presidential-electioneering wrinkle. This strategy of avoiding Congress to push through radical changes to American industry, transport and power production was designed by senior Obama advisor John Podesta, who is likely to leave the administration shortly to become chairman of Hilary Clinton’s 2016 White House bid. These targets are not just Obama’s targets; they are Clinton’s too. And the US media is reporting that Podesta chose this strategy to make sure that the 2025 targets could be achieved without any of Obama’s successors having to pass legislation through a future Congress. This explicit, acknowledged avoidance of the legislative process does open Clinton up to a powerful line of attack that the Democratic Party is afraid of democracy, the ideal the party is named after.

In the end, as Corinne Le Quéré, the director of the Tyndall Centre for Climate Change Research, points out, the optimism from the deal comes from the fact that we seem to have turned the corner on the very worst climate outcomes: “The best these commitments may be able to do is move the world away from the extremely high levels of climate change that we are currently on track for, leading to 3-5 degrees warming, levels that are threatening world food security and economies.”

“This is already a relief, but it is nowhere near the warming limit of two degrees Celsius that these same governments pledge to adhere to,” she says, speaking to Road to Paris.

But the real significance of the US-China deal is its diplomatic breakthrough rather than the actual pledges. One of the major impasses in climate negotiations has been overcome. In some respects the deal mirrors the climate agreement between Washington and New Delhi announced in September that established a new U.S.-India nuclear power partnership after years of deadlock and furnished $1 billion in fresh financing from the U.S. Export-Import Bank to help with the purchase of American clean-energy technology.

The deal also ramps up the pressure on other states, in particular Japan, Canada and Australia, all major emitters who have pulled back in recent years from earlier mitigation commitments. They now appear a little bit lazy compared to their traditional climate allies in Brussels and Washington.

The UN process foresees all countries making their initial pledges by Spring 2015, yet already, before the end of 2014, we have modest opening bids from Europe, the US, and China and an understanding of the path India intends to take. The headline targets may not be sufficient to avoid two degrees of warming, but at least four of the biggest players are all now moving in broadly the same direction.

Crucially, five years after the Copenhagen debacle, we can allow ourselves a glimmer of hope.