Earth may experience a radically different climate already within 34 years, forever changing life as we know it, said a study Wednesday that aims to bring the dangers of global warming into sharper focus.

On current trends of greenhouse-gas emissions, 2047 will mark the year at which the climate at most places on Earth will shift beyond documented extremes, it said.

This date is pushed back to 2069 under a scenario in which fossil-fuel burning emissions are stabilised, said an analysis of climate projections published in the journal Nature.

"The results shocked us," lead author Camilo Mora of the University of Hawaii's geography department said of the findings.

"Within my generation, whatever climate we are used to will be a thing of the past."

Most climate studies predict average, global shifts by a randomly-chosen cutoff date like 2100.

The new study took a different tack by distinguishing between different areas of the world, and seeking to identify the year in which climate change will cross the threshold where weather events once viewed as extreme become the norm.

It looked at effects such as air and sea-surface temperature, rainfall and ocean acidity.

"Regardless of the scenario, changes will be coming soon," said Mora -- forcing species to adapt, move or die out.

"The work demonstrates that we are pushing the ecosystems of the world out of the environment in which they evolved into wholly new conditions that they may not be able to cope with. Extinctions are likely to result," commented Ken Caldeira of the Carnegie Institution for Science's global ecology department.

The tropics may be hit soonest and hardest, according to the study.

Tropical plants and animals are not used to variations in climate and are thus more vulnerable to even small changes.

"The tropics hold the world's greatest diversity of marine and terrestrial species and will experience unprecedented climates some 10 years earlier than anywhere else on Earth," said a statement.

They are also home to the bulk of the world's population and contribute significantly to global food supply.

"In predominantly developing countries, over one billion people under an optimistic scenario, and five billion under a business-as-usual scenario, live in areas that will experience extreme climates before 2050," study co-author Ryan Longman said.

"This raises concerns for changes in the supply of food and water, human health, wider spread of infectious diseases, heat stress, conflicts and challenges to economies.

"Our results suggest that countries first impacted by unprecedented climates are the ones with the least capacity to respond."

Under a business-as-usual emissions scenario, the team predicted dates of "climate departure" ranging from 2020 for Manokwari in Indonesia, 2029 for Lagos in Nigeria and 2031 for Mexico City, to 2066 for Reykjavik in Iceland and 2071 in Anchorage, Alaska.

"Climate departure" is the point at which extremes measured over the past 150 years, the period for which weather data is considered reliable, become the norm.

"If the assessment... proves accurate, conservation practitioners take heed -- the climate-change race is not only on, it is fixed, with the extinction finish line looming closest for the tropics," Eric Post of Pennsylvania State University's biology department wrote in a comment on the findings.

The date of 2047 is based on a "business-as-usual" scenario under which atmospheric levels of carbon dioxide (CO2) continue unabated.

Currently just under 400 parts per million, they would reach 936 ppm by 2100, meaning an average rise in temperature over this century of around 3.7 degrees Celsius (6.6 degrees Fahrenheit).

The date of 2067 is based on a slowing of emissions, which would reach 538 ppm by 2100, giving warming this century of around 1.8 C (3.24 F).

About 0.7 C (1.3 F) must be added to these temperatures to include warming that occurred from the start of the Industrial Revolution to 2000.

The United Nations has set a target of limiting warming to 2 C (3.6 F) over pre-industrial levels.