Of all the explanations for Barack Obama's Nobel peace prize, the one that rang truest came from Nicolas Sarkozy. "It sets the seal on America's return to the heart of all the world's peoples." In other words, this was Europe's way of saying to America, "We love you again", like those weird renewal-of-vows ceremonies couples have after a rough patch.

Now Europe and the US are officially reunited, it seems appropriate to consider whether this is necessarily a good thing. The Nobel committee, which awarded the prize for Obama's embrace of "multilateral diplomacy", is evidently convinced that US engagement on the world stage is a triumph for peace and justice. I'm not so sure. After nine months in office, Obama has a clear track record as a global player. Again and again, US negotiators have chosen not to strengthen international laws and protocols but to weaken them, often leading other rich countries in a race to the bottom.

Let's start where the stakes are highest: climate change. During the Bush years, European politicians distinguished themselves from the US by expressing their unshakable commitment to the Kyoto protocol. So while the US increased its carbon emissions by 20% from 1990 levels, European Union countries reduced theirs by 2%. Not stellar, but clearly a case where the EU's break-up with America carried tangible benefits for the planet.

Flash forward to the high-stakes climate negotiations that have just wrapped up in Bangkok. The talks were supposed to lead to a deal in Copenhagen this December that significantly strengthens Kyoto. Instead, the developed countries formed a bloc calling for Kyoto to be replaced. Where Kyoto set clear and binding targets for emission reductions, the US plan would have each country decide how much to cut, then submit its plans to international monitoring – with nothing but wishful thinking to ensure this all keeps the planet's temperature below catastrophic levels. And where Kyoto put the burden of responsibility squarely on the rich countries that created the climate crisis, the new plan treats all countries the same.

These kinds of weak proposals were not altogether surprising coming from the US; what was shocking was the sudden unity of the rich world around the plan – including many countries that had previously sung the praises of Kyoto. And there were more betrayals: the EU, which had indicated it would spend between $19bn and $35bn a year to help developing countries adapt to climate change, came to Bangkok with a much lower offer, one more in line with the US pledge of … nothing. Oxfam's Antonio Hill summed up the talks like this: "When the starting gun fired, it became a race to the bottom, with rich countries weakening existing commitments under the international framework."

This isn't the first time a much-celebrated return to the negotiating table has resulted in overturned tables, with hard-won international laws and conventions scattered on the floor. The US played a similar role at the United Nations conference on racism in April. After extracting all sorts of deletions from the negotiating text – no references to Israel or the Palestinians, nothing on slavery reparations – the Obama administration decided to boycott anyway, pointing to the fact that the new text reaffirmed the document adopted in 2001 in Durban.

It was a flimsy excuse, but there was some kind of logic to it, since the US had never signed the 2001 agreement. What made no sense was the wave of copycat withdrawals from the rich world. Within 48 hours of the US announcement, Italy, Australia, Germany, the Netherlands, New Zealand and Poland had pulled out. Unlike the US, these governments had all signed the 2001 declaration, so they had no reason to object to a document that reaffirmed it.

It didn't matter. As with the climate change talks, lining up behind Obama – with his impeccable reputation – was an easy way to avoid burdensome obligations and look progressive at the same time: a service the US was never able to provide during the Bush years.

The US has had a similarly corrupting influence as a new member of the UN human rights council. Its first big test was Judge Richard Goldstone's courageous report on Israel's Gaza onslaught, which found that war crimes had been committed by both the Israeli army and Hamas. Rather than prove its commitment to international law, the US used its clout to smear the report as "deeply flawed" and to strong-arm the Palestinian Authority into withdrawing a supportive resolution. The PA, which faced a furious backlash at home for caving in to US pressure, may introduce a new version.

And then there are the G20 summits, Obama's highest profile multilateral engagements. At the April meeting in London, it seemed for a moment there might be some kind of co-ordinated attempt to rein in transnational financial speculators and tax dodgers. Sarkozy even pledged to walk out of the summit if it failed to produce serious regulatory commitments. But the Obama administration had no interest in genuine multilateralism, advocating instead that countries should come up with their own plans (or not) and hope for the best – much like its reckless climate-change plan. Sarkozy, needless to say, did not walk anywhere but to the photo session, to have his picture taken with Obama.

Of course, Obama has made some good moves on the world stage – like not siding with the Honduras coup government, or supporting a UN women's agency. But a clear pattern has emerged: in areas where other rich nations were teetering between principled action and negligence, US interventions have tilted them toward negligence. If this is the new era of multilateralism, it is no prize.