The British TV audience for the Live Earth concerts was less than a third of that for last week's Concert for Diana.
BBC One's average ratings between 2000 and 2200 BST on Saturday were 3.1 million, compared with 11.4 million for the Diana gig on the previous Sunday.
Bad language by stars attracted more than 130 complaints to the BBC - but more than 400 complained about rock band Metallica's set being cut short.
The BBC said the ratings were "good" for "a Saturday night in July".
"The audience for Live Earth peaked at nearly five million viewers," a spokeswoman said.
"Climate change is an important and challenging issue which the BBC will continue to engage with."
Swearing
Madonna used foul language when she encouraged fans at Wembley Stadium to "start jumping up and down" as a way to "save the planet".
Genesis singer Phil Collins swore while adapting the lyrics to one of his songs, causing BBC Radio 1 DJ Chris Moyles to apologise for his "potty mouth" afterwards.
And the BBC cut away from an introduction by US comedian Chris Rock after he swore on air.
The BBC was "sorry for any offence" caused by swearing and its presenters had been "quick to apologise", the spokeswoman said.
"It was technically all-but-impossible to broadcast with a delay. It was called Live Earth and it was always intended to be broadcast live."
Acts had been asked to avoid using bad language, "but sometimes they got carried away", she said.
'Poor' sound
Metallica were seen playing Sad But True and Nothing Else Matters.
But BBC presenter Jonathan Ross interrupted their set to introduce a recording of Weather With You by Crowded House from Live Earth in Sydney in place of the Metallica song Enter Sandman.
Some 413 people complained about the decision.
The BBC spokeswoman claimed the concert was "running behind schedule" and producers wanted to insert the Crowded House video at that point "to reflect what was happening across the globe".
Several people who were at the concert at Wembley Stadium also contacted the BBC News website to say they were unhappy about the sound quality inside the venue.
They had concerns about a time delay in hearing the bands, with sound from different speakers scattered around the stadium reaching fans at different times.
Live Earth concerts were staged in nine locations including Tokyo, Johannesburg, New Jersey and Rio de Janeiro.
They were inspired by former US Vice-President Al Gore and were intended to raise awareness of climate change.
Critics argued the gigs themselves caused pollution, but organisers said they were as environmentally-friendly as possible.
There were also claims that many of the celebrities involved were poor role models because they flew around the world with entourages while on tour.
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