"We hope there will be a strong political commitment by the leaders attending this meeting," said Ban, noting he was aware of "mixed feelings" among the negotiators from nearly 200 countries.
About 100 ministers and a handful of heads of state gathered in Doha on Tuesday for the final, high-level stretch of the talks marked so far by bickering over cash and commitments needed to curb greenhouse gases.
Observers say delegates remain far apart on issues vital for unlocking a global deal on climate change.
Poor countries insist that Western nations sign up to deeper, more urgent cuts in carbon emissions under a follow up, second commitment period for the Kyoto Protocol, and agree to a new funding package from 2013 to help them cope with worsening drought, flood, storms and rising seas.
Ban told delegates attending a briefing on the sidelines of the talks that climate change "is approaching fast, much, much faster than we expect".
He cited superstorm Sandy, which struck the US east coast and the Caribbean last month, as a "call to action that before it is too late we have to take action."
Ban is due to address the full meeting at around 1200 GMT.
The Doha negotiators, he said, must reaffirm the world's commitment to a follow up for Kyoto, which runs out on December 31, to reaching a new, universally-binding climate pact to enter into force by 2020, and funding climate mitigation projects in the developing world.
"I know there are some mixed feelings, optimism and pessimism, but we have to work on the basis of optimism. If there is no optimism there is no result. That is what I am asking you."
The UN chief said that during his stay in Doha, he would meet groupings of countries as well as individual countries to "discuss how we can work together".
"We have a responsibility. We have a moral responsibility and as ministers and leaders you have political responsibility for our future generations," he said.
"Please remember that we are loaning our future from our next generation."