The government's proposals to tackle climate change need to be tougher and legally enforceable, say MPs and peers.
MPs say people need to accept "the realities of climate change"
Their report said the target of a minimum 60% cut in carbon emissions by 2050 may not be adequate.
International aviation emissions should be included in targets and there should be a cap on the use of "carbon credits" to meet them, the joint committee said.
The government said it was on track to beat its targets and the aim of a 60% cut was being kept under review.
The report backed the government's five-yearly carbon targets, but said there should be annual "milestones" and reports. Both the Tories and the Lib Dems have called for annual carbon targets.
The joint committee on climate change was examining the government's draft Climate Change Bill - which ministers say shows Britain is "leading by example".
It sets out plans to reduce carbon emissions by a minimum of 60%, from the 1990 base level, by 2050 - and sets an interim target of "at least 26% but not more than 32%" by 2020.
The committee said that the 32% upper limit on carbon reductions should be removed, as there was no "compelling reason" for it.
And it expressed "surprise" that the government intended to buy foreign carbon credits to meet 70% of its emission savings under the EU emissions trading scheme.
It says there should be an "absolute cap" on their use, saying: "The bill as currently drafted would still theoretically allow all the savings to be made externally to the UK, notably in developing countries, and thereby postponing the decarbonisation of the UK economy."
The joint committee said it was a "serious weakness" that emissions from international aviation were not included in the targets and says the government is not taking sufficient account of predicted growth in aviation emissions.
"Further thinking" on legal enforceability of targets and budgets was needed, it said, and it stressed the need for sufficient powers, resources and independence for the proposed oversight body, the Committee on Climate Change.
And the report says the government must give a higher priority to changing the behaviour of individuals with major public information campaigns.
The committee's chairman, Labour's Lord Puttnam, told BBC Radio 4's Today programme there would have to be a "quite significant shift in behaviour change".
But he welcomed the "groundbreaking" draft bill and dismissed arguments that Britain's contribution mattered little, when other, larger countries continued to pollute.
Friends of the Earth campaigns director Mike Childs said the draft bill had to be strengthened.
"The government must listen, it must include international aviation in the emissions reductions targets and it must set a higher target to cut emissions based on the latest scientific evidence," he said.
Martin Harper, head of sustainable development at conservation charity the RSPB, said the target should be 80% not 60% to stop temperatures rising too quickly.
But Environment Secretary Hilary Benn said the UK should be proud of its record on tackling climate change and that it was on track to go beyond its targets.
"Government must rightly lead in combating climate change, and that's why we will put this ambitious legislation in place," he said.
Mr Benn added he would reflect on the committee's comments but a 60% commitment was "ambitious by any standard and consistent with our leading position internationally".
However, the 60% goal would be under review in the light of emerging scientific evidence, he said.