The Taoiseach, Mr Ahern, has said that the task of the World Summit on Sustainable Development is to "pick up the pace" and realise the vision of the first Earth Summit in Rio de Janeiro 10 years ago.
"Johannesburg must initiate the decade of action on sustainable development", he told a plenary session.
"It matters for the many, many millions who are poor and starving. It matters for our children and for future generations. Let us not fail in this historic task".
Mr Ahern was speaking after negotiators had concluded a deal on the last remaining contentious issue facing the summit - energy. But he made it clear that Ireland remained "adamantly opposed" to any expansion of the nuclear industry, saying it should have no role in sustainable development.
Much of his speech concentrated on the plight of poorer countries, as exemplified by the threatened famine facing 13 million people in southern Africa, and he described the decline in overseas development aid since Rio as "shameful, indefensible and inconsistent with the commitments" made there.
The Taoiseach reiterated Ireland's "absolute commitment" to achieving the UN target of spending 0.7 per cent of GDP on overseas development aid by 2007, saying the increase of €100 million in the Budget for this year was "more than the value of our entire aid programme a few years ago".
He also said the Government supported "in principle" the cancellation of the debts of heavily indebted poor countries. This would be an important contribution by donor countries to the New Partnership for Africa's Development, an economic and political alliance of countries on the continent.
"We are acutely conscious of the depth of human suffering, and the threat of famine, in the immediate neighbourhood," Mr Ahern told the summit. What had made this food crisis even more serious was that the countries involved were also bearing the brunt of the HIV/AIDS epidemic.
He also said the poor and most vulnerable were least equipped to adapt to environmental change. At the same time, unsustainable patterns of production and consumption persisted in the developed world, where economic growth had yet to be decoupled from environmental degradation.
Referring to Ireland itself, the Taoiseach said the Government attached a high priority to environmental management and protection.
"We are also gearing up to meet our Kyoto commitment and prepare for the tougher action that is necessary to tackle climate change", he declared.
At a briefing afterwards for the Irish media, Mr Ahern said it was essential to meet the requirement under the Kyoto Protocol to cap the increase in Ireland's greenhouse-gas emissions to 13 per cent above their 1990 levels by 2010, a target that will be difficult to reach without firm action.
He also said that the UN Secretary-General, Mr Kofi Annan, whom he met yesterda, had been impressed by Ireland's commitment to overseas development aid, particularly the extra allocation of €100 million for 2002 "even though times are a bit tougher", as the Taoiseach said.
The Fine Gael spokesman on foreign affairs, Mr Gay Mitchell, strongly criticised the Taoiseach's promise to commit 0.7 per cent GDP to developmental aid as "a callous PR stunt in light of the Government's recent cut in the overseas aid budget".
"How exactly does the Taoiseach think he can reconcile his promises with his Government's actions?
"The simple answer is that he can't, because it is impossible. He must face up to his responsibilities and admit to another broken promise," Mr Mitchell said.
As Irish NGOs were forced to advertise for donations on a daily basis, the Minister of State, Mr Tom Kitt "had the nerve to visit areas devastated by famine and disease, knowing that he has betrayed all of those who live there".
The international community could not take the Government's latest promises seriously, nor could the Irish people or the parliament.
"The Taoiseach must make amends immediately by enacting legislation to secure a fixed contribution level. Fine Gael will, of course, facilitate the passing of any such Bill through the Oireachtas," Mr Mitchell said.