By Richard Black
Environment correspondent, BBC News
Mr Aso described the target as "extremely ambitious"
Japan has announced a target of cutting greenhouse gas emissions by 15% over the next 11 years - a figure derided by environmentalists as "appalling".
The target equates to a cut of about 8% from 1990 levels, the commonly used baseline. By comparison, the EU plans a 20% reduction over the same period.
The announcement comes in the middle of talks on the UN climate treaty in Bonn.
Some observers say Japan's goal is not enough to persuade developing countries to cut their own emissions.
"The target is not strong enough to convince developing nations to sign up for a new climate change pact," said Hidefumi Kurasaka, professor of environmental policies at Japan's Chiba University.
Announcing the target, Prime Minister Taro Aso argued it was as strong as the EU's because it does not include "flexible mechanisms" such as international carbon trading.
But Kim Carstensen, leader of the global climate initiative at environment group WWF, said the 8% target represented virtually no advance from the 6% cut that Japan had pledged, under the Kyoto Protocol, to achieve by 2012.
"Prime Minister Aso's plan is appalling," he said.
"[It] would mean that Japan effectively gives dirty industries the freedom to pollute without limits for eight years."
Japan's annual emissions are currently about 6% above 1990 levels, despite its Kyoto Protocol pledge to make cuts.
But the government points out that the society uses energy much more efficiently than other industrialised countries. Per-capita greenhouse gas emissions are about half the rates in Australia and the US.
Last year, Mr Aso's predecessor Yasuo Fukuda set a longer term target of cutting emissions by 60-80% by 2050, and indicated the 2020 target would be close to the EU's.
Japan is a leader in technologies such as solar power
To the chagrin of environment groups - who point the finger at lobbying from Japanese industry - this has not transpired.
EU environment commissioner Stavros Dimas welcomed Japan's commitment, but suggested it had much further to go.
"Japan should consider further ways and means to make up the difference so that it can get us in the range recommended by science," he said in a statement.
The Intergovernmental Panel on Climate Change (IPCC), the UN advisory body, has recommended that developed nations cut emissions by 25-40% from 1990 levels by 2020.
Mr Aso's vision puts Japan roughly in line with the US and Australia in setting less ambitious targets.
President Obama has pledged to bring emissions back down to 1990 levels by 2020, although legislation coming through Congress is likely to impose a target of about 6%. Australia is aiming for cuts of 5-15% on 2000 levels by 2020, although the top end of that only applies if other nations pledge major cuts.
Speaking at the UN climate convention meeting in Bonn, the convention's executive secretary Yvo de Boer said industrialised countries' pledges fell short of levels indicated by the science.
"We're still a long way long way from the ambitious emission reduction scenarios of the IPCC that are a kind of beacon in terms of what industrialised countries need to do if we are to avoid the worst impacts of climate change," he said.
But, he commented, the list of commitments was not yet complete. There were different baselines, he said, different usage of flexible mechanisms, and some countries (notably the US and Russia) have yet to set definite goals.
"The challenge that we face from here on is to get the list complete, and then get into a discussion on comparability of effort, see how we can increase the level of ambition and continuously keep in the corner of our eye that beacon from the IPCC in terms of the scientific necessity."
The two-week meeting, which ends on Friday, is the latest in a series leading up to December's key summit in Copenhagen, which is supposed to usher in a climate agreement to supersede the Kyoto Protocol, whose current emissions targets only run as far as 2012.
The US, and some EU nations, are determined that major developing countries such as China and India should adopt emission curbs.
But they have repeatedly said they will not sign up to measures that could curb their economic growth, arguing that the developed world must lead the way.