Barack Obama has warned that the climate crisis "will force far harsher changes" on future generations than the coronavirus pandemic and urged people to "demand more" from leadership to protect the planet.

In a message on Twitter on Earth Day, the former president shared an Associated Press report about the young climate activists that have galvanised a global movement 50 years after Earth day was recognised, and how the youth-led movement has adapted to the current health crisis.

He said: "All of us should follow the young people who've led the efforts to protect our planet for generations, and demand more of our leaders at every level."

His successor Donald Trump meanwhile marked Earth Day's 50th anniversary by planting a tree outside the White House and declaring the US to have "the cleanest air and cleanest water than anywhere else on Earth" — it does not.

The nation's air quality has been in decline for several years, scientists and researchers have reported, including the American Lung Association's recent State of the Air report that found nearly half of the US, roughly 150 million people, is breathing polluted air.

This year also marks the 50th anniversary of the Clean Air Act, which the Environmental Protection Agency has cited for most of the emissions reductions within the last few decades. But the Trump administration has proposed dramatic rollbacks to the law or dropped enforcement entirely.

The State of the Air report notes that 2016, 2017 and 2018 — years with the most recent air quality data — were also among the five hottest recorded years in history. Changing climate patterns "fuel wildfires and their dangerous smoke, and lead to worsened particle and ozone pollution".

American Lung Association president Harold Wimmer said that the 2020 report "shows that climate change continues to degrade air quality and increase the risk of air pollution harming health. To protect the advances in air quality we fought for 50 years ago through the Clean Air Act, we must again act today, implementing effective policies to protect our air quality and lung health against the threat of climate change."

On Tuesday, the White House also finalised rule changes to Obama-era protections in the Clean Water Act to effectively remove limits on the amount of pollution that can be released into streams and wetlands. The rule change could eliminate protections for roughly 81 per cent of streams in the US southwest.