Announcement at climate summit comes after former US secretary of state John Kerry warns that international action is slowing

California is set to launch a satellite to track greenhouse gases, as former US Secretary of State John Kerry and island nation leaders warned that the world is far off course to avoid the worst effects of rising temperatures.

Gov. Jerry Brown announced plans for the satellite on the last day of a climate change summit hosted by San Francisco, in a final rebuke to President Donald Trump’s denial of man-made warming.

“With science still under attack,” Brown said “we’re going to launch our own satellite, our own damn satellite, to figure out where the pollution is.” Brown said the satellite will help pinpoint the source of planet-warming emissions.

California will team up with Planet Labs, a company run by ex-Nasa scientists. The data collected, including on carbon dioxide emissions and methane leaks from oil and gas operations, could be made public as part of a partnership with the advocacy group Environmental Defense Fund. The new project comes as Trump has proposed slashing Nasa climate research mission budgets. It is one of dozens of commitments of mixed significance unveiled by states, cities and businesses at the event.

Despite the optimism on show at the summit, Kerry said climate efforts must ramp up.

“I am going to tell the truth, and the truth is we are not anywhere near where we need to be with respect to the overall challenge of climate change,” said Kerry, who worked to secure the 2015 global Paris climate agreement under former president Barack Obama.

Kerry blasted Donald Trump for deciding to leave that deal, calling it “one of the single greatest acts of irresponsibility by a president of the United States anywhere at any time.”

Leaders of the countries already suffering most from sea-level rise and ocean acidification echoed Kerry’s concerns, saying that international action is slowing.

“The world has lost, all of us have lost, momentum since Paris in 2015. Although the rate of increase has slowed, we’ve not yet peaked our global emissions. But we must do so by 2020. We really cannot afford to wait any longer,” said Mia Mottley, prime minister of the Caribbean island nation of Barbados.

Mottley’s country is in the direct path of hurricanes that are growing in strength and may narrowly avoid a more direct hit from tropical storm Isaac this week.

The world is set to watch temperatures rise 3C above pre-industrial levels by the time a child born today is old, Mottley said, even if countries adhere to the goals they said.

Frank Bainimarama, prime minister of Fiji, said countries need to speed their work.

“We all know that the levels of ambition in our national plans need to be ramped up because we are not on track to meet the targets of the Paris agreement,” Bainimarama said.

Former US vice-president Al Gore struck a more positive tone.

“We must do it. We can do it. I’m convinced ever more because of the success of this summit here in San Francisco that we will do it,” he said, reminding that the US has not technically left the Paris deal yet and that a new president could re-enter.

The warnings were at odds with the overall atmosphere of the summit.

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On the eve of the gathering, California governor Jerry Brown signed a bill that would make the state’s electricity supply carbon-free by 2045. A separate executive order by Brown is more sweeping, committing to net zero emissions across the entire California economy, also by 2045.

Other cities and regions from around the world have followed this with various pledges, with New York City promising $4bn to renewable energy and clean water and cities including Los Angeles, Tokyo, Honolulu, Oslo and Greater Manchester pledging to build energy efficient buildings or deploy fleets of electric buses.

A group of 29 philanthropists committed $4bn over five years to combat climate change, the largest such investment of its kind, while companies such as Ikea, Walmart and Unilever promised to reduce emissions through measures such as electrified trucks for deliveries and action to prevent deforestation in the tropics.

Jonathan Pershing, the State Department’s climate negotiator under Obama, said the summit brings hope to the climate cause.

“The story here is optimistic. The question here is does the optimism translate, and can this message get out globally,” Pershing said. “There is a good broad cross-section of people from around the world, but it’s just a few thousand people, and it’s a problem that’s going to require engagement by millions.”