US President Joe Biden promised to halve his country's greenhouse gas emissions by 2030 from 2005 levels at a global climate summit to pressure other polluters to bring their emissions in line with international agreements.

"The cost of inaction keeps mounting. The United States isn't waiting," he said on Thursday in the opening address of the two-day summit hosted virtually by the White House. He also pledged to double US contributions to global climate finance by 2024 and triple funding for adapting to its effects.

"No nation can solve this crisis on its own, and this summit is a step on a path to a secure, prosperous, and sustainable future," he said.

Several other countries also upped their pledges.

Japan announced plans to cut emissions 46% below 2013 levels by 2030. Canada said it would aim to slash emissions 40%-45% by 2030 below 2005 levels. China's President Xi Jinping reiterated his country's goal to peak CO2 output before 2030 and achieve carbon neutrality by 2060, adding that coal use would gradually decrease in the second half of this decade.

Brazil's Jair Bolsonaro pledged to go climate neutral by 2050, bringing his country's goal forward by 10 years, and end illegal deforestation, which has spiked under his leadership, by 2030.

The pledges were met with protests from climate strikers across the world demanding immediate action instead of "empty promises."

"You need to accept that the era of fossil fuels is over," said Xiye Bastida, a youth climate activist from Mexico, addressing the leaders on Thursday. "We demand that you stop systematically targeting the Global South — and Black, brown and indigenous communities — through environmental plunder."

Ambitious climate targets are 'very encouraging'

The US has dirtied the atmosphere with carbon dioxide more than any other economy and its citizens have some of the highest carbon footprints in the world today. Though the 50% target announced on Thursday is not binding, it raises the US commitment to cutting emissions by about a third.

World leaders in 2015 promised to limit global warming to well below 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial temperatures in a bid to hold off the worst effects of climate change. But as recently as November, a United Nations report found their official plans to achieve this were "woefully inadequate."

"More would be needed to meet the 1.5-degree limit," said NewClimate Institute director Niklas Höhne, referring to the new US emissions goal.

While more ambitious climate targets are "very encouraging indeed," said Prakash Kashwan, a professor of political science who researches climate justice at the University of Connecticut. Big emitters like the US and EU should explicitly recognize the carbon debt they owe the rest of the world instead of using their domestic actions to cajole other countries. "Lead from the front by delivering actual emission reductions."

Climate ambition

The 40 world leaders who met virtually for Biden's two-day summit are responsible for 80% of the yearly greenhouse gas emissions that are heating the planet and wreaking havoc in the form of worse droughts, cyclones, floods, heatwaves and wildfires.

As well as the big polluters, leaders from some of the countries least responsible but most vulnerable to climatic shifts also attended, like the Democratic Republic of Congo, Jamaica and Bangladesh.

Some countries announced steeper cuts in the run-up to the meeting, which is seen as a critical milestone on the road to the COP26 UN climate summit to be held in Glasgow, UK, in November.

On Tuesday, the UK set the world's most ambitious target with a law binding the country to cut greenhouse gas emissions 78% by 2035 from 1990 levels. The goal includes emissions from ships and flights abroad that have traditionally been ignored by domestic governments. Later that evening, EU lawmakers reached a legally binding deal to make the bloc's climate goal — to cut net greenhouse gas emissions 55% by 2030 from 1990 levels. The target is less ambitious than the 60% the European Parliament had previously voted for.

Climate diplomacy

US climate envoy John Kerry met his Chinese counterpart Xie Zhenhua in Shanghai last week as part of a renewed push in US climate diplomacy. But while they agreed to discuss concrete actions this decade to cut carbon pollution, they did not announce more ambitious pledges.

"For low- middle-income countries, we need to see both China and the US are committing," said Marcelo Mena-Carrasco, director of the Climate Action Center at Pontificia Universidad Catolica de Valparaiso and a former Chilean environment minister. "That's a way to see the global burden of emissions is taken care of."

A similar trip to India also failed to yield stronger commitments. During the meeting, the Indian Foreign Ministry stressed the need for rich countries to fulfill their unmet pledge to send $100 billion (€120 billion) a year in climate finance to poorer countries by 2020. The call was repeated by several world leaders at the summit.

As part of an executive order in his first days in office, Biden announced a new climate plan — released during the summit — on how the United States will work with the World Bank and International Monetary Fund to help poorer countries cut their emissions and protect ecosystems.

But even delivering on that promise may not be enough to convince poorer countries to give up fossil fuels their economies so heavily rely on.

"Climate finance is such a small bucket it doesn't support transformational changes," said Jennifer Tollmann, a senior policy advisor at Berlin-based climate think tank E3G. 

Speaking on the second day of the summit, Microsoft co-founder Bill Gates called on governments and businesses to adopt policies that will make it faster and cheaper to transition away from fossil fuels. "Leaders will need to reward those that take difficult steps."

Broken promises

Biden also used the summit to portray the US as a global leader in the fight against climate change. Its reputation had suffered under the administration of his predecessor Donald Trump, who in addition to taking the US out of the international Paris Agreement also rolled back environmental laws while promoting coal, oil and gas.

The US "has offered nothing on how it plans to make up for the lost four years," said Chinese Foreign Ministry Spokesperson Zhao Lijian at a press conference on Friday. "Its return is by no means a glorious comeback but rather the student playing truant getting back to class."

Chinese Premier Xi Jinping surprised the world in September by announcing that China would peak its carbon dioxide emissions by 2030 and reach net-zero by 2060. But it has since continued to invest in fossil fuel projects, particularly to build coal-fired power plants across Asia.

South Korea, meanwhile, announced at the summit that it will stop funding coal plants overseas.

As well as finances and emissions, the summit also hosted sessions on jobs, climate security, adaptation and nature-based solutions that include further efforts to stop deforestation.

A report published Tuesday by the Rainforest Foundation Norway found that none of the five countries with the biggest rainforests — Brazil, Democratic Republic of Congo, Indonesia, Peru, and Colombia — have committed in their emissions plans to end deforestation by 2030, despite having agreed to do so.

A common frustration among activists and scientists watching the talks was the lack of commitment to immediate action.

"We have been having the same song with other leaders, so we are tired of empty promises," said Kevin Mtai, a climate activist with student protest group Fridays for Future in Kenya, adding that Biden should immediately stop US-headquartered fossil fuel companies causing environmental problems in the Global South. "We need action not later but right now, because the planet is dying."

This article was updated throughout the summit.