We know President Trump is holding China's feet to the fire on trade, but now he is playing tough on climate issues too.

While media reports from the G20 summit have emphasized the Trump administration’s insistence on a summit communique that noted Trump’s decision to withdraw from the Paris Agreement, the statement flap was far from the biggest climate story in Osaka.

Indeed, the hour-long meeting between Trump and Chinese President Xi Jinping — or, more precisely, the evolving and tumultuous U.S.-China relationship — seems likely to be considerably more consequential for international cooperation in addressing climate change.

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Because the Paris Agreement is voluntary, not mandatory, its value depends largely on the practical outcomes that it yields over time. To be a success, Paris must not merely generate compliance with its terms, but also either establish a foundation for far deeper internationally-agreed reductions in greenhouse gas emissions, as many people hope, or build a lasting framework to accelerate the discovery and deployment of more affordable low- and zero-emissions energy technologies.

It is also possible to envision a future in which humans stop and reverse climate change without the Paris Agreement. It is much harder to see such a future without a workable global system to develop and share key technologies.

Since the United States and China have the world’s two largest economies, and are not coincidentally the two biggest emitters, whether and how Washington and Beijing deal with their many differences will have profound implications for both international climate diplomacy and the day-to-day realities of energy technology research, investment and trade. That is why the fundamentals of the U.S.-China relationship are probably more important than a G20 document expressing continuing commitment to a non-binding international agreement that Trump has already vowed to abandon.

Recent signs have not been good. The on-again off-again U.S.-China trade war, in an off-again phase following the Trump-Xi meeting, is producing daily economic harm to both sides; some estimate the costs to the United States at 0.1-0.2 percent and to China at 0.3-0.6 percent of gross domestic product (GDP).

From the long-term perspective required to deal with a multi-decade challenge like reducing greenhouse gas emissions, unwinding economic, educational, scientific and other connections that the United States and China took over 40 years to build may be no less consequential than the back-and-forth over tariffs. Some senior administration officials explicitly seek a "decoupling" of the two economies.

Moreover, U.S.-China trade tensions are not merely a result of economic complaints. Washington officials and U.S. businesses are deeply frustrated with China’s theft of intellectual property, a complaint that strikes at the core of the global research, development and investment that will underlie efforts to deploy new energy technologies. American officials and politicians are no less troubled at the prospect of incorporating Chinese components into critical infrastructure, which has been most visible in the restrictions that the Trump administration imposed and then eased regarding Huawei and its telecommunications business. Less known is that a bipartisan group of Senators has asked the Department of Energy to ban Huawei’s solar inverters.

If the U.S and Chinese markets for zero-emissions energy systems are mutually closed, or sharply restricted, it cannot but slow efforts to build and use the systems—especially if Washington continues efforts to discourage America’s allies from working with Huawei and other Chinese companies.

Also problematic in the longer term are signs that U.S. attitudes toward China may be changing. While caution is always appropriate in drawing enduring conclusions from a single survey, a February 2019 poll by the Chicago Council on Global Affairs demonstrated a striking increase in the share of Americans who view China as primarily a rival rather than a partner, from a steady 47-49 percent over the last 13 years to nearly two-thirds (63 percent) today.

If sustained, this national shift in perspectives toward China could likewise have lasting effects that go well beyond trade and diplomacy. Former House Speaker Newt Gingrich has warned of the consequences of this in recent commentary.

While the U.S. research establishment is doubtless a target for China’s intelligence services, it is not hard to envision how a changing political and media climate in the United States could transform 350,000 Chinese students in America into suspects, much as Russia’s election interference prompted suspicion of virtually all Russians.

Some thoughtful observers have argued persuasively that U.S. pressure is necessary to change China’s problematic behavior and that, if successful, a strategy of pressure and negotiation could avert decoupling.

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This is a pragmatic and reasonable view. Nevertheless, the realities of differing U.S. and Chinese interests, growing mutual suspicion and resentment, and modern-day nationalist politics — in both countries — may add momentum to the erosion of U.S.-China political, economic and social ties. Even if leaders on each side ultimately want no more than a relationship that they think is just a little bit better for their people, this momentum might prove quite hard to slow, much less to redirect.

To promote deeper U.S.-China cooperation in combating climate change, advocates will have to demonstrate that the benefits for America of working with China, in both economic and climate dimensions, will be greater than the possible cost in U.S. jobs, competitiveness, and security that come with the transfer of wealth and technology from the United States to China. In America’s emerging politics, this will be no easy task for anyone — Democrat or Republican.