The historic climate deal signed in Paris last December has come into force after 94 countries ratified the agreement last month. The United Nations warns that huge emissions cuts are now needed to meet the global goals.
United Nations climate chiefs said humanity would look back on Friday as the day the world "shut the door on inevitable climate disaster and set off with determination towards a sustainable future."
"Its early entry into force is a clear political signal that all the nations of the world are devoted to decisive global action on climate change," they said.
Fifteen years to act
Espinosa, along with Mezouar - who is also Morocco's foreign minister - warned that the world community would need to come together within the next 15 years to ensure "unprecedented reductions in greenhouse gas emissions and unequalled efforts to build societies that can resist rising climate impacts."
They warned that emissions are not yet falling, with carbon dioxide (CO2) levels breaking new records in 2016. Concentrations of CO2 in the atmosphere passed a critical and symbolic threshold of 400 parts per million last year, the UN said.
By 2030, emissions are expected to reach 54-56 billion tonnes of carbon dioxide equivalent, far above the level of 42 billion tonnes needed to have a chance of meet one of the key components of the Paris accord - limiting global warming to 2 degrees Celsius (3.6 degrees Fahrenheit) this century.
Among the questions will be how to best disburse $100 billion (90 billion euros) a year to poor, climate-vulnerable nations.
While the conference takes place, American voters will decide their next president, with many of the 15,000 COP22 attendees watching intently to see if Republican nominee Donald Trump - who has publicly denied climate change is taking place - makes it to the White House.