By Bridget Kendall
BBC diplomatic correspondent
Spring is a time for renewal and optimism.
In London, as the days lengthen and the sun peeps out, British officials are trying to manage high hopes that April's G20 summit will somehow find a global financial cure.
President Obama addressed a video message to the Iranian people
But anticipation also hovers over another policy realm - nuclear security and the chance of a thaw in US-Iranian relations.
The focus is on President Barack Obama and the policy review he has ordered. Although Tehran insists its nuclear aims are peaceful, the US and its partners suspect a secret plan to develop nuclear weapons - with the potential to shake the Middle East and reawaken a global nuclear arms race.
So the West still calls on Iran to suspend its enrichment programme. And US sanctions against Iran have just been extended. But, at the same time, President Obama is making overtures.
There's the offer of direct talks if Iran "unclenches its fist"; there's the attempt to reach out to Iran on regional issues - inviting it to a big Afghanistan conference due shortly, and planning to send a US official to the next Shanghai Co-operation Council meeting of Asian countries in Moscow, which Iran will also be attending.
And now a new video message - with Farsi subtitles - from Barack Obama has assured Iranians that the US wants "engagement that is honest and grounded in mutual respect", prompting a swift response from Iran's Supreme leader.
As usual Ayatollah Khamenei is scathing about the US and lays down Iranian conditions, demanding an end to America's sanctions and its unquestioning support of Israel.
Ayatollah Khamenei responded to the US overture by demanding policy changes
But note that he also said that if the US really did change, then Iran would change its behaviour too. There is a new mood in the air, a sense that something is moving.
One related American shift has been towards Russia. Mr Obama's administration does not hide the fact that it sees continued US-Russian tensions as counterproductive. Better to sort out quarrels with Moscow than risk having disgruntled Russians hampering efforts to deal with the bigger challenge of Tehran's ayatollahs.
So there is now talk of a possible "grand bargain". A private letter from President Obama to President Dmitry Medvedev seems to have thrown down the gauntlet. The buzz is that Washington might offer to hold off its controversial missile defence programme in Europe, in return for new Russian leverage on Tehran.
It's unclear what might be asked of Moscow: perhaps support for a broader economic embargo against Iran if negotiations fail, perhaps to lay off further arms sales - half promised already - which would strengthen Iran's air defences.
Who knows what will emerge? After all, the Russians are likely to try to push for a tough bargain. They have their red lines when it comes to European security, and their public posture at least is that it should be Washington that makes the main concessions.
But whatever is being planned, there is no time to waste. For several reasons the Iran problem is urgent.
A test run has been carried out at Iran's Bushehr nuclear power plant
First, if it is true Iran wants to show the world it can make a bomb, the clock is ticking.
Despite an infamous US intelligence report two years ago, which claimed any Iranian threat was still distant, the latest report from the UN's nuclear watchdog, the IAEA, suggests Iran already has or may soon acquire a stock of enriched uranium large enough - in theory - to grade up into a small weapon.
Secondly, there is a mounting consensus that the damage done by casting Iran as a member of the "axis of evil" needs to be undone if there is to be any chance of restoring trust on either side. Even some US diplomats who helped spearhead Bush's policy on Iran admit to failure.
"We had advocated regime change. We had a very threatening posture towards Iran for a number of years. It didn't produce
any movement whatsoever," said Nicholas Burns, former senior diplomat in the US State Department, in a documentary broadcast on the BBC recently.
Thirdly, if the Bush administration began by forging policy unilaterally and ended by swinging tactically on Iran, then President Obama needs to think broadly and act strategically.
Given the burdens - military and financial - that the US still shoulders in Iraq and increasingly Afghanistan, any new approach which spreads the load or which turns Iran from a headache into a help when it comes to stabilising its neighbours, must be an advantage.
And with four - or possibly eight - years of his presidency ahead of him, Barack Obama has to face the fact that it'll probably be on his watch that any Iran crisis reaches a climax.
Any Iran crisis is likely to reach a climax on Mr Obama's watch
Will he be the American president who opts for military attacks on Iran as a last resort, to keep his word that an Iranian bomb would be "unacceptable"?
Or will he be the president who finally ends 30 years of US/Iranian animosity and pulls the world back from the brink of a new atomic escalation?
Two stark choices.
But multi-lateral engagement and radical rethinks are back in fashion.
The current financial crisis requires it. So do the looming dangers of climate change. How to tackle the threat of a nuclear arms spreading is woven into this web of global challenges.
Britain's Gordon Brown last week made the link succinctly. As the world tries to wean itself off carbon-spewing fossil fuels, demand for nuclear energy will grow, he argued.
Thirty-two new nuclear reactors a year will be needed for the next four decades to reach the goal of halving carbon emissions, according to UN estimates.
But with that comes the risk of ever more nations using their civilian nuclear plants to conceal secret military programmes, and of ever larger stockpiles of fissile material and nuclear waste falling into the hands of rogue states or terrorists.
France, along with Britain, has cut its nuclear arsenal
Britain's vision is for a new tough global regime, where nuclear energy is shared internationally, but where controls are tightened and sanctions for rule breakers made harsher and swifter.
Iran, said Gordon Brown, would be a "test case" for this "new philosophy": either agree to collaborate and reap the benefits, or refuse to play ball and face greater penalties.
A "refreshed grand nuclear bargain for our age", Gordon Brown calls it.
But as Britain, America and others contemplate how to harness the genie of nuclear proliferation, now it is out of the bottle, they must surely admit that at the heart of any arrangement lies an uncomfortable inequality.
This is the increasingly out-of-date imbalance between the "haves" and the "have-nots" in the nuclear weapons club. Why is it that the five signatories to the Non Proliferation Treaty who already have nuclear weapons - the US, Russia, China, Britain and France - are allowed to keep them, but any other countries that sign up to the treaty must renounce the prospect?
Good for world peace, but not exactly fair. Especially when the NPT stipulates that these five are obliged to work towards reducing their stockpiles eventually to zero.
That has not been the message of the past few years.
Both Britain and France have cut their arsenals. Britain says it has cut the number of its warheads by half since 1997, but that did not stop the government deciding two years ago to upgrade its Trident nuclear submarine fleet.
Russia's focus too has been on upgrading its nuclear missiles. Just last week President Medvedev announced that the economic crisis would not stop Russia re-equipping the entire armed forces, with priority given to nuclear arms.
Then there is Israel, long suspected of holding nuclear weapons, under little or no pressure from Western allies to declare its hand or disarm
China is thought to be developing new missiles and warheads. And under George W Bush, the United States gave no leadership on this. His Pentagon tried to get funding for a new generation of bunker-busting nuclear missiles.
No wonder the last two reviews of the NPT treaty in 2000 and 2005 ended in dismal failure.
Add to this the fact that those non NPT nations that have tested nuclear weapons (India, Pakistan and North Korea), far from being punished, have found themselves rewarded and their negotiating position immediately strengthened. Then there is Israel, long suspected of holding nuclear weapons, under little or no pressure from Western allies to declare its hand or disarm. From Tehran's point of view, it is easy to claim a double standard.
Given all that, how do you persuade a country like Iran, eager to be seen as the dominant power in the region, not to succumb to the temptation of acquiring nukes, or at least reach the tipping point where it can use them as a bargaining chip?
The US needs the co-operation of Russia's Medvedev
The answer, surely, must lie not only in carrot and stick bargaining, or step by step strengthening of bilateral ties, but to set an example.
Look first to the big five.
In this era of budget cuts, perhaps cutting arms spending is even appealing. Already President Obama is on record as saying he supports the idea of one day eliminating nuclear weapons. Between them, the US and Russia hold some 95% of the world's nuclear weapons. And they have just agreed to start discussing a new arms deal to replace the START 1 Treaty which expires in December. There is hopeful talk of a ceiling of 1,000 war heads. Though there'll no doubt be plenty of hard wrangling.
And Britain has chipped in - indicating it would be willing to look at further cuts if indeed the world moves towards multilateral disarmament.
Perhaps the next NPT review in 2010 might come up with real progress.
But I would argue there is a further rethink needed.
Nuclear weapons are not just about capabilities, they are about prestige - a seat at the top table. Who are the five veto-wielding members of the UN Security Council? The five original nuclear powers. UN Security Council reform might also help send a message that global clout does not just rest with those who have armed themselves with atomic warheads.
Of course, none of this addresses the biggest question of all: if Iran's intentions are indeed peaceful, why has it not already agreed to discuss the deal being offered by the West, which includes help in establishing a civilian nuclear industry as well as the lifting of economic sanctions?
What if those in charge in Tehran are only playing for time, and do not view improved relations with the outside world as an enticement? Iran's people may welcome greater engagement, but do its clerical leaders really want to open Iran's doors to the US and run risk of their own power being diluted, both economically and politically?
Even if they insist it would be un-Islamic and illegal, what if their private calculation is that nuclear blackmail is the best way of staying in power and extending Iran's influence regionally and globally?
Which brings us back to the nightmare scenario.
"What happens if Iran won't negotiate?" I asked one senior British diplomat recently. "What's Plan B?"
"Don't go there," he replied wearily, "It's too difficult, and too frightening."
You can listen to the BBC World Service's The Forum with Bridget Kendall