US Secretary of State John Kerry on Thursday issued a stark warning about the mounting effects of climate change, from food shortages to devastating droughts, saying time was running out for real action.

Kerry said the window for facing the challenge was "closing quickly" and warned of dire consequences if climate change skeptics are wrong about the future and nothing is done.

"If they're wrong, catastrophe," Kerry said in Boston after visiting a wind-technology testing center with British Foreign Secretary Philip Hammond.

"Life as you know it on Earth ends. Seven degrees increase Fahrenheit (4 degrees Celsius) and we can't sustain crops, water, life under those circumstances."

Kerry, who has long touted environmental causes, said technological solutions could still prevent worst-case climate disasters.

"The solution is staring us in the face. It's very simple: clean energy," he said, noting the prospects for creating millions of jobs worldwide in the sector.

"And here is the kicker," Kerry said, "the market we're looking at is a $6-trillion market with four to five billion users today, climbing to a potential nine billion users by the year 2050. It is literally the mother of all markets."

Kerry warned that climate change already is taking a toll, noting that this past August was the hottest in the planet's recorded history.

He said food shortages in places like Central America were occurring because of the worst droughts in decades and warned that rising sea levels could wreak havoc to low-lying cities and communities.

"Think about it just in terms of Boston," he said, referring to a predicted one-meter (3.2-feet) rise in sea level by century's end.

"It would mean about $100 billion worth of damage to buildings, to emergency costs and so on."

Kerry said the world needs to act on climate change -- while it still can.

"We still have in our hands a window of opportunity to be able to make the difference," he said.

"But the window is closing quickly. That's not a threat, that's a fact."

Just as President Barack Obama did during last month's United Nations Assembly General in New York, Kerry urged for a worldwide agreement on climate change to be reached during an international conference planned in Paris at the end of 2015.

Last month, world leaders vowed to redouble efforts to combat climate change, but the pledges fell short.

Some $2.3 billion was pledged to the Green Climate Fund, but the amount was well below the $10 billion aimed for by year-end -- let alone the aimed-for $100 billion a year starting in 2020 to help poor nations cope with rising temperatures and disasters.