"The number of victims caused by climate change is very big... bigger than the victims of wars," said the voice, whose authenticity could not be immediately verified and made available by SITE Intelligence Group.
The tape would be the first time Bin Laden has spoken publicly since March 25.
It was not clear when the tape was made, but bin Laden congratulated Muslims on the September 10 end of the holy fasting month of Ramadan.
"The catastrophe (in Pakistan) is very big and it is difficult to describe it," he said.
"What we are facing... calls for generous souls and brave men to take serious and prompt action to provide relief for their Muslim brothers in Pakistan."
Bin Laden involved in attack on European cities: report
Earlier, National Public Radio reported that al Qaeda chief Osama bin Laden is involved in an unfolding plot to launch attacks on European cities and may have even targeted the United States.
Several months ago, bin Laden sent a directive to al Qaeda affiliates and partners that he wanted a Mumbai-style attack on at least three European countries -- the United Kingdom, Germany and France -- National Public Radio said, citing intelligence officials and people familiar with the matter.
NPR said gunmen had planned to fire on crowds at busy European tourist sites and take over hotels in a plot that would mark a new style of attack for al Qaeda, although details of the plans remain unclear for now. The United States may also have been in bin Laden's sights.
"We know that Osama bin Laden issued the directive," an unnamed official familiar with intelligence surrounding the plot told NPR. "And if he issued the directive, we just don't believe that the US wouldn't be on his short list of strategic targets. It has to be."
The public radio station said initial intelligence came from Ahmad Siddiqui, a German national currently held at the US-run Bagram Air Base in Kabul, Afghanistan. Siddiqui is said to have known Mohamed Atta, one of the alleged hijackers in the September 11, 2001 attacks, and to have worshipped at the same mosque in Germany.
Intelligence officials told NPR some of the operatives who had been due to participate in the shootings were already in Europe. Some officials worried that members of the commando-style teams could be travelling to the West using European passports, thus complicating any effort to find and stop them.
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