A crucial milestone toward entering the Paris climate accord into force was achieved this week, as the world's two largest emitters of greenhouse gases (GHGs) officially ratified the treaty. President Barack Obama of the United States and President Xi Jinping of China delivered their signatures to United Nations Secretary-General Ban Ki-moon on Saturday on behalf of their respective governments. Both leaders had telegraphed the widely anticipated move ahead of time, with the G20 Summit in Hangzhou, China, presenting an ideal forum for a joint announcement. A senior Chinese official had gone on record in April stating China's intent to ratify before the Summit, while the White House deployed personnel to China for climate policy discussions in advance of the meeting. Ratification of the Paris accord marks a major environmental and foreign policy victory for the Obama Administration and represents a shining beacon within the sometimes cloudy relationship between the two superpowers of late. Many developed and developing nations alike have indicated they were waiting on China and the United States to ratify the treaty before taking action themselves. The significance of adding both nations in a unified, unequivocal manner cannot be understated and will likely spark a series of ratifications heading into 2017.

In order for the Paris accord to enter into force, a minimum of 55 countries representing 55 percent of total global GHG emissions must take action for "ratification, acceptance, approval or accession," all of which indicate the country's agreement to participate in the treaty. Some nations must meet additional requirements back home, such as obtaining the approval of their national legislature or enacting supporting or authorizing legislation. For instance, China's National People's Congress granted approval for ratification prior to President Xi's signature. Other countries, like the United States, can use executive authority for ratification, which is why President Obama did not require a vote in Congress before acting. As with most international treaties, the Paris accord is a political agreement rather than a legally binding document. Thus, the United States did not require any new legislation in order to ratify it. Instead, President Obama's executive power to join and implement the treaty have previously been granted under the Clean Air Act and Energy Policy Act of 1992 (technically making the Paris accord an executive agreement, rather than a traditional treaty).

The addition of the United States and China to the accord represents a giant step toward entry into force in terms of represented emissions and political motivation. Twenty-five other states have joined the accord at the time of this writing; their 1.1 percent combined share of total global emissions pales in comparison to the nearly 38 percent share attributed to the world's two largest economies. The addition of the United States and China orients a substantial portion of the global market toward reducing GHG emissions in fulfillment of those country's INDCs (Intended Nationally Determined Contributions, the climate mitigation goals set by each signatory state). Private sector investment in energy efficiency, renewable energy technologies, alternative fuels, vehicle electrification, research and development, and myriad other mitigation-oriented projects could skyrocket in service to a significantly greener global economy. Public and private stakeholders risk being left behind if they fail to plan for the new post-Paris market.

The G20 announcement reassures the other major industrialized nations they won't have to face the challenges of another international climate treaty alone. The American-Chinese ratification also sends a strong signal to developing nations that the parties responsible for the vast majority of GHG emissions driving climate change are serious about implementing solutions. Meanwhile, other nations continue to move forward on ratification. Brazil's Senate ratified the treaty in August, with the state's interim president vowing to sign the deal in September. Japan and New Zealand have indicated a desire to join as well, and may do so by the end of 2016. Back in North America, Mexico is laying the groundwork for a national cap and trade scheme and recently announced an agreement with Canada to begin integration with regional carbon markets in the provinces of Ontario and Quebec. The European Union's central body has been vocal in its desire to expedite ratification and is currently coordinating the complex task of ratification by each of its 28 individual member states, with France ratifying in June to become the first major industrialized nation to do so. The United Kingdom's newly installed Prime Minister has also gone on record in favor of ratification. A combination of the aforementioned signatory states joining would be more than enough to reach the Paris accord's emissions threshold of 55 percent, leaving only the 55 country threshold to reach for entry into force.

Despite a sometimes tense relationship over foreign affairs, defense, and trade, the United States and China have quietly built a productive partnership around climate change. Heading into the G20 Summit, the two governments reportedly agreed to share fossil fuel subsidies data to facilitate peer review of domestic energy policies. The agreement marks the first time China has permitted a foreign government to directly review its domestic energy subsidies and opens the door to greater transparency into China's regulatory practices. The peer reviews are an extension of a 2009 consensus by G20 leaders to begin phasing out fossil fuel subsidies, but concrete results have been lacking. China and the United States have also expressed mutual support for an international cap on carbon emissions for the civil aviation industry and are collaborating to phase out HFCs (hydrofluorocarbons, a potent GHG used in refrigeration) under an amendment to the Montreal Protocol. These joint actions promise to support the broader goals of the Paris accord and may encourage other countries to follow the lead of the American and Chinese governments in addressing the factors behind global climate change.