* World's two biggest emitters ratify global climate accord
* Ratification could push agreement into legal effect by year-end
* U.S., China pledge support for aviation emissions pact
HANGZHOU, China, Sept 3 (Reuters) - The United States has joined China to formally ratify the Paris agreement to curb climate-warming emissions, the world's two biggest economies said on Saturday, which could help put the pact into force before the end of the year.
U.S. President Barack Obama and Chinese President Xi Jinping submitted their plan to join the agreement to U.N. Secretary-General Ban Ki-moon, who is in China to witness the announcement.
Senior Obama adviser Brian Deese said the joint declaration should push other countries to formally join the agreement.
"The signal of the two large emitters taking this step together and taking it early, far earlier than people had anticipated a year ago, should give confidence to the global communities and to other countries that are working on their climate change plans, that they too can move quickly and will be part of a global effort," Deese told reporters on Friday.
India is also poised to join the agreement this year, Deese said, adding that Obama was expected to meet Indian Prime Minister Narendra Modi on the sidelines of a Group of 20 nations meeting in Hangzhou, China, this weekend.
Obama and Xi committed to cooperate on two other global environmental agreements this year - an amendment to the Montreal Protocol to phase down air-conditioning refrigerants and on a market-based measure to reduce carbon emissions from aviation.
"Today's announcement is the strongest signal yet that what we agreed in Paris will soon be the law of the land," said Mattlan Zackhras, minister-in-assistance to the president of the Marshall Islands.
"With the two biggest emitters ready to lead, the transition to a low-emissions, climate-resilient global economy is now irreversible."
Saturday's joint statement could spur further ratifications by the likes of Brazil and Canada.
"We expect a surge of ratifications around the U.N. Climate week later in September," said Bill Hare, chief executive of Climate Analytics.
In Paris last December, nearly 200 countries agreed on a binding global compact to slash greenhouse gases and keep global temperature increases to "well below" 2 degrees Celsius.
Experts have said the temperature target is already in danger of being breached, with the U.N.'s weather agency saying 2016 is on course to be the warmest year since records began.
While 180 countries have now signed the agreement, 55 nations - covering at least 55 percent of global emissions - need to formally ratify the treaty to put it into legal effect.
Before China and the United States, 23 nations had ratified - including North Korea - but they collectively accounted for just 1.08 percent of global emissions, according to the U.N. Framework Convention on Climate Change.
China represents just over 20 percent of global emissions while the United States accounting for 17.9 percent, Russia 7.5 percent and India 4.1 percent.
The announcement is a major diplomatic achievement for the U.S. president, who ends his term in January.
But the ability of the United States to achieve its Paris targets could be affected by the outcome of a federal court hearing this month, in which 27 U.S. states are trying to block the federal Clean Power Plan that slashes CO2 from power plants, the largest source of U.S. greenhouse gas emissions.
The U.S. Republican Party Platform has also questioned the legality of the executive order used to ratify the Paris deal, saying it will need the consent of the Senate before it becomes binding.
Li Shuo, a climate adviser with Greenpeace, said both China and the United States were determined to put the treaty into force as soon as possible in order to avoid the risk that any new Republican administration would reject it.
"It now looks like the Paris agreement will enter into force before the end of the year and that will really be light speed compared to almost all other international agreements," he said.
U.S. Democratic presidential candidate Hillary Clinton is a strong supporter of the accord, but her Republican counterpart Donald Trump has dismissed man-made climate change as a hoax and says he will abandon the Paris agreement if elected.
Countries that ratify the deal will have to wait for three years after it has gone into legal force before they can begin the process of withdrawing from it, according to the agreement signed in Paris.
Ratification, however, does not mean the work is over.
Alden Meyer, international director of the Union of Concerned Scientists, said the Paris agreement's detailed rules will likely take another year or two to finalise.
"All countries will need to raise the ambition of their commitments under the agreement if we're to avoid the worst impacts of climate change and reach a goal of net zero global warming emissions by mid-century," Meyer said.
"But this is an important step forward that reinforces the U.S. and China's continued leadership in building a robust, durable international climate framework." (Reporting by Valerie Volcovici and Ayesha Rascoe in WASHINGTON, Alister Doyle in OSLO, David Stanway in SHANGHAI; Writing by David Stanway; Editing by Robert Birsel)