By Richard Black
Environment correspondent, BBC News website
Rich nations had to lead emissions cuts, he said, but developing countries such as China should have targets too.
Mr Thomas also told BBC News that too little money was available to help poor countries prepare for climate impacts.
Britain will next week launch a major international study to identify the best ways to fund climate adaptation.
Britain and its European allies hope the Bali meeting will begin a process that will lead to a further international set of binding targets for emissions cuts to take effect when the current Kyoto Protocol targets expire in 2012.
A major sticking point in the UN process has been whether big developing countries such as China, India and Brazil should take on firm commitments.
The US in particular argues that with China's emissions set to overtake and eventually exceed its own, there is little point in making a deal that includes only developed nations.
"I frankly think every developing country has to have clear targets," said Mr Thomas.
"Some developing countries who are at a very low stage of development are going to have to be allowed to increase their emissions, while the bigger developing countries draw back on theirs, and developed countries draw back even faster on their carbon emissions."
However, he said developed nations could not escape their primary responsibility for climate change.
"Quite understandably, (developing countries) are looking to developed nations such as ourselves and the US and the rest of the EU to take more of the burden of responsibility, and we in the UK have accepted that we have that role to play."
Adapt and survive
The Bali meeting is also looking at adaptation - assisting developing countries to protect their societies and economies against climate impacts such as hotter growing seasons, drought, floods and disease.
The British and Dutch governments will be announcing a two-year project in six developing countries to research the best ways of funding adaptation.
The research will focus on analysing the resources already available, how much extra donor assistance is needed, ways of drawing in the private sector, and on assessing how much money is needed each year.
"Frankly we don't know the scale of the financial challenge," said Mr Thomas.
"Various estimates have been put forward - anything from $11bn to $86bn (£5.5bn to £42bn) has been touted as being necessary."
The idea was endorsed by Oxfam, one of the charities heavily involved in climate adaptation.
"We've previously called for a global audit for adaptation - Oxfam believes the sum needed will be around $50bn (£25bn) a year.
"But the fact that they're taking adaptation seriously and doing research doesn't mean we should take a breather and wait for the results - people need to start funding adaptation immediately."
Earlier this week, Oxfam, together with other charities and the UN Development Programme, poured scorn on the scale of the global commitment to adaptation funding - "less than Americans spend on suntan lotion each month" and "roughly what Britain spends on flood defences each week" were two of the unflattering comparisons.
Gareth Thomas agreed that the sums available fall well short of what was needed.
"I can understand the frustration," he said.
"[But] I do think we need to get our information right, it will help to shape how we approach the private sector, how we look for innovative financial solutions that involve the private sector in helping to address deforestation, etc."
They key to generating adaptation funds, the government believes, is a global and effective carbon market.
But many economists believe such a market cannot function properly without a global system of emission caps.The British and Dutch governments will formally launch their research fund during the Bali meeting