G20 leaders send a symbolic message on a key climate target.

Leaders of the Group of 20 nations sent a symbolic message on Sunday as one of the most important climate conferences began, pledging to “pursue efforts” to keep the average global temperature rise to 1.5 degrees Celsius by the end of this century.

While the mention of the number, seen as a critical threshold for limiting the severest effects of climate change, is a step forward, the leaders did not say how their countries would reduce their emissions more aggressively to achieve that goal.

“We remain committed to the Paris Agreement goal to hold the global average temperature increase well below 2°C and to pursue efforts to limit it to 1.5°C above pre-industrial levels, also as a means to enable the achievement of the 2030 Agenda,” the leaders said in a statement

Saying they “look forward to a successful” climate conference, the leaders said, “We recognize that the impacts of climate change at 1.5°C are much lower than at 2°C.”

However symbolic the commitment, it was not without significance, noted Prime Minister Mario Draghi of Italy. “Now, for the first time, all the countries of the g20” acknowledge the scientific merit of the 1.5.-degree goal, he said.

The scientific consensus is that if the average global temperature rises by 1.5 degrees Celsius — 2.7 degrees Fahrenheit — it will significantly increase the likelihood of far worse climate catastrophes that could exacerbate hunger, disease and conflict. That consensus came in a landmark report a few years after the Paris agreement was reached in 2015, which had set the goal at “well below” 2 degrees Celsius.

The language of the statement sends an important signal to the United Nations-led international climate summit that began in Glasgow on Sunday. Its host, Britain, and the United States have made the 1.5-degree goal something of a rallying cry.

G20 nations account for the vast majority of the local greenhouse gas emissions warming the planet, and they hold the key to averting the worst consequences of global warming.

“Keeping 1.5°C within reach,” the leaders said in their statement Sunday, “will require meaningful and effective actions and commitment by all countries, taking into account different approaches, through the development of clear national pathways that align long-term ambition with short- and medium-term goals, and with international cooperation and support, including finance and technology, sustainable and responsible consumption and production as critical enablers, in the context of sustainable development

At the moment, however, achieving a 1.5-degree cap is a highly ambitious goal.

Even if all countries achieve the targets they set for themselves in the Paris Agreement, average global temperatures are on track to rise by 2.7 degrees Celsius by the end of the century. Reaching the target would require big polluting countries to strengthen those targets, or Nationally Determined Contributions, as they are known, by committing to reduce emissions much faster between now and 2030.

The leaders committed “to take further action this decade” and to update their plans as necessary. .