TIMELINE-How U.N. climate conferences have tackled global warming

By Andrea Januta

Nov 2 (Reuters) - This year’s U.N. climate conference in Sharm el-Sheikh, Egypt, marks the 27th time since 1995 that world leaders have gathered to confront global warming. But the world has known for far longer that climate change was a threat, and that the cause was mainly fossil fuel use and other industrial activity.

Here are some key moments in the global climate conversation:

1800s - Throughout the 1800s, several European scientists study how different gases and vapours can trap heat in the Earth’s atmosphere. In the 1890s, Swedish scientist Svante Arrhenius calculates the temperature effect of a doubling of atmospheric CO2, showing that burning fossil fuels would likely warm the planet.

1938 - By compiling historical weather data, British engineer Guy Callendar for the first time shows the planet's temperatures are rising in the modern era. He correlates the temperature trends with measured rises in atmospheric CO2 and proposes the temperature change is linked.

1958 - American scientist Charles David Keeling starts systematically measuring atmospheric CO2 levels over Hawaii’s Mauna Loa Observatory. His findings result in the “Keeling Curve,” a graph showing CO2 concentrations steadily increasing.

1988 - James Hansen, an American climate scientist, testifies before Congress that the planet is warming because of a human-caused buildup of greenhouse gasses and notes that this is already altering the climate and weather.

1990 - At the U.N.’s so-called Second World Climate Conference, scientists highlight the risks of global warming to nature and society. British Prime Minister Margaret Thatcher says binding emissions targets are needed.

1992 - Countries sign onto the U.N. Framework Convention on Climate Change at the Rio Earth Summit. The UNFCCC’s goal is to control emissions to prevent extreme climate change, but it also enshrines the idea of “common but differentiated responsibilities,” meaning developed countries must do more because they are responsible for most historical emissions. The treaty does not set out binding emissions targets.

1995 - UNFCCC treaty members gather for a first “conference of parties,” or COP, in Berlin. The final document calls for legally binding emissions targets.

1997 - At COP3 in Kyoto, Japan, parties agree to the first treaty that requires specific emissions reductions. Under the Kyoto Protocol, developed countries are obligated to reduce emissions between 2008 and 2012 from 1990 levels, with different limits assigned to different countries. In the United States, key Senate Republicans denounce the accord as “dead on arrival.”