The huge crowd turnout at the "Claim the Climate" march in Brussels on a wet Sunday afternoon came as a big surprise to me because the Belgian capital has a population of just a little more than 1 million.

The Cinquantenaire Park, where the protesters finally gathered, was packed with people of all ages, worried about their own future and that of their children and grandchildren. Local media estimated about 70,000 people attended the march.

In an awakening report by the UN Intergovernmental Panel on Climate Change in October, the world's top climate scientists warned that there is only a dozen years left to keep global temperature rise to within 1.5C－beyond which each 0.5C increase will significantly worsen the risks of droughts, floods, extreme heat and poverty for hundreds of millions of people on the planet. The scientists called for urgent and unprecedented changes to reach the climate target.

The demonstrators in Brussels were responding to the IPCC call in urging their government and the world to take immediate action against climate change. The march was organized to coincide with the beginning of the UN Climate Conference in Katowice, Poland, where leaders and representatives of some 200 countries are meeting to help put the Paris Agreement into real action.

The Paris climate accord, adopted in December 2015, set a global action plan to put the world on track to avoid dangerous climate change by drastically reducing emissions while pursuing efforts to limit the temperature rise to within 1.5C.

UN Secretary-General Antonio Guterres has sent many tweets on climate change in the past weeks. On Tuesday, he announced to hold a UN Climate Summit in New York during the week of the UN General Assembly in September next year with the goal of raising real ambition, and promoting transformative action in the real economy and an unprecedented citizen and youth mobilization.

There is no doubt that US President Donald Trump's decision last year to pull out the US from the Paris Agreement has greatly hurt the cause. The US was the largest greenhouse emitter before 2006 and its per capita emission is still more than twice that of China.

Last month, Trump even cast doubt on the Fourth National Climate Assessment, a report by his own government in which experts warned of the devastating effects of climate change. The report lists the adverse impact of climate change on US communities, economy, agriculture, water, infrastructure, health, indigenous people and coasts.

"I don't believe it," Trump reportedly said in response to the report.

In a recent article, Jeffrey Sachs, a professor at and director of the Center for Sustainable Development at Columbia University, said that those leaders who oppose action to address human-induced climate change should be held accountable for climate crimes against humanity.

In Poland on Monday, former California governor Arnold Schwarzenegger told the audience that Washington's leadership is "a little bit backward" and people should look at US cities and states which are taking actions against climate change.

China and India are major supporters of the Paris climate accord. But the two largest developing nations also face huge challenge in fighting climate change. In a report on Wednesday, the World Health Organization estimated that 1 million lives could be saved through reduction in air pollution alone. The benefits would be higher in countries key to tackling global emissions, such as China and India, according to the WTO report.

There is no doubt that all countries have much to do to combat climate change before it's too late.

The author is chief of China Daily EU Bureau based in Brussels.