Two of the country's most volatile fault lines straddle San Francisco, where thousands of leaders gathered last week for the first-ever Global Climate Action Summit. The event laid bare two growing chasms in the nation's war on climate change.

The most obvious chasm is the rift between President Trump's administration and its critics, many of whom attended the summit. The day before the summit began, the Environmental Protection Agency proposed to rollback rules against methane leaks from oil and gas. California Gov. Jerry Brown quickly denounced the rollback as insane, dangerous and irresponsible. But the proposal was just the latest dagger through the heart of President Obama's climate initiatives, as Trump moves to renounce the Paris Climate Agreement, halt fuel economy improvements and repeal and replace the Clean Power Plan.

Governors, mayors and corporate leaders at the summit pledged their renewed resolve to uphold the Paris agreement. As Houston Mayor Sylvester Turner put it, if the president "doesn't lead from the top, mayors will lead from where they are." Houston's first-ever climate action plan was one of over 500 commitments issued by cities, states, businesses and universities ahead of the summit.

California took the lead with a flurry of bills and an executive order signed by Gov. Brown just before the summit began — one bill committing the state to 100 percent clean electricity by 2045, matching a date set by Hawaii. Brown also signed legislation to strengthen incentives for zero-emission vehicles, expand vehicle charging infrastructure and improve the efficiency of homes and buildings.

Gov. Brown then issued a moonshot pledge — an executive order for California to become carbon neutral by 2045. That means that any emissions from cars, factories and other sources would need to be balanced by storing carbon in forests, soils and other sinks.

It's no understatement to call Gov. Brown's pledge "history's most ambitious climate target." It's also unclear how California will achieve it. Perhaps it won't, since executive orders don't carry the force of legislation. The bill eliminating power plant emissions will get California only one-sixth of the way there. Emissions from other industries and vehicles are far more difficult to clean up. Carbon sequestration technologies are still in their infancy, and there's little reason to deploy them without a hefty price on carbon.

Pledging carbon neutrality without a plan to get there highlights the second chasm in the war on climate change — the widening gap between ambitions and achievements. Last week's summit described itself as an opportunity to "celebrate the extraordinary achievements" of states, cities and businesses and serve as a "launchpad" for deeper commitments.

In fact, it was the commitments that were truly extraordinary, while actual achievements continue to lag ambitions.

U.S. emissions have fallen 12 percent since 2005 after decades of inexorable growth. But that downward momentum is grinding toward a halt, as Trump's moves counteract state and local actions. Analysts project emissions will bottom out around 17 percent below 2005 levels, hovering far above our Paris Agreement pledge for a 26 percent to 28 percent cut by 2025.

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Just before the summit began, the America's Pledge initiative released a report assessing 10 climate action strategies that could narrow that shortfall. Those strategies include raising targets for renewable energy and electric cars, making homes and businesses more efficient, stopping methane leaks, soaking up carbon with forests and croplands and putting a price on emissions.

Each hypothetical strategy was designed to be achievable by states and others without relying on federal action. But it's hard to imagine all 10 being achieved any time soon, let alone the additional "enhanced engagement" measures that would still be needed to push down U.S. emissions "within striking distance" of the 2025 target. That's like layering wishful thinking on top of hopes and prayers — and still coming up a little bit short.

Even if they succeed, emissions would still be falling at just over one percent per year. That pace would need to zoom up by a factor of five to reach Obama's subsequent target for an 80 percent reduction by 2050.

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The 2050 target is just another milestone toward cutting net emissions to zero, as the entire world must do to hold global warming below 2 degrees Celsius (3.6 degrees Fahrenheit). Carbon pollution accumulates in the atmosphere for centuries, so our climate will keep warming until it can equilibrate with stable carbon levels. In other words, carbon neutrality must be the ultimate aim if we take our temperature targets seriously.

That's why the Paris Agreement signatories and Governor Brown have set carbon neutrality as their ultimate ambitions, and why America's Pledge sees such urgency in meeting our initial targets. But those ambitions far outpace actual achievements. Climate Action Tracker projects that under current policies, U.S. emissions will stop falling and global warming will top 3 degrees Celsius (5.4 degrees Fahrenheit) this century and continue rising thereafter.

Last week demonstrated the continued resolve of governors, mayors and executives to fend off such bleak projections, even as the federal government abandons its leadership. They'll eventually need to join forces to match ambitions with achievements in battling climate change.

Daniel Cohan is an associate professor in the Department of Civil and Environmental Engineering at Rice University.

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