Four years ago, President Barack Obama warned President-elect Donald Trump on his visit to the Oval Office after the election that the biggest national security threat he would face was North Korea's impending ability to fire nuclear missiles against the United States.

If such a visit between president and president-elect were to happen now, Trump could warn Joe Biden about not just one but three nuclear crises in the coming years.

Not only has North Korea continued to grow its nuclear missile inventory, but Iran has increased production of nuclear materials tenfold since Trump withdrew from the nuclear deal in 2018. And the last remaining nuclear arms control agreement with Russia is set to expire within weeks of Biden taking office.

The nuclear threats are but one among a host of pressing issues confronting Biden. Even so, the Biden administration can ill afford to ignore any of them.

The most pressing is extending the New START treaty with Russia, which is set to expire Feb. 5. Talks with Moscow on extending the treaty have stalled over Trump's demand for additional restrictions. Though these demands are sensible — including a verifiable freeze on the number of all nuclear warheads (strategic and nonstrategic, deployed and stored) — it would be much better to negotiate any new provisions after the New START has been extended for five years. Such an extension would, at the very least, keep the arms control framework that has governed U.S.-Russian nuclear relations intact.

The second pressing nuclear issue concerns Iran. In 2018, Trump made good on his campaign pledge to withdraw from the Iran nuclear agreement. But while he reimposed punishing economic sanctions, the decision also freed Tehran to restart its nuclear program.

In recent days, the UN's nuclear agency reported that Iran had produced almost 2,500 kilograms of enriched uranium, sufficient to build two nuclear bombs, and also restarted advanced machinery to speed up the production process. These reports may explain why Trump recently asked his senior advisers for options to take out Iran's nuclear facilities. Though the president seems to have been persuaded that such a strike risked a wider war, the Iran nuclear question remains a pressing concern.

Biden has pledged to return to the original deal, but that won't be easy. Not only will some of the key provisions in the original deal phase out in a few years, but Iran will likely exact a steep price for returning to compliance, including compensation for the costs of the new sanctions. And in both countries, political opposition to returning to the deal is bound to intensify.

Biden, however, has little choice but to try and negotiate binding limits on Iran's nuclear capabilities. A nuclear Iran would be dangerous for the world. The 2015 agreement was hardly perfect, but it was far preferable to the unconstrained growth of Iran's nuclear program that we're once again confronting.

The dangers of unrelenting nuclear proliferation are all too apparent in North Korea, the third nuclear crisis confronting Biden. Although Trump and Kim Jong Un met three times and exchanged glowing letters over the past two years, the pleasantries did nothing to curtail North Korea's nuclear ambitions. Pyongyang has continued to churn out nuclear bombs and new missiles at a fast clip, and last month it showed off new sea- and land-based missiles likely capable of reaching all of U.S. territory.

For 30 years, the U.S. and its Asian allies have insisted on the complete and verifiable denuclearization of the Korean Peninsula. That is still the right goal.

But the lesson over five administrations is that we won't get there in one fell swoop. We need a new incremental approach that starts with freezing current capabilities, extends to closer political negotiations (including a peace treaty), adds arms control measures and builds trust over an extended period. Only then is denuclearization a realistic possibility.

These three nuclear crises, if not addressed with urgency, could well come to dominate Biden's entire presidency.

Ivo Daalder is president of the Chicago Council on Global Affairs and a former U.S. ambassador to NATO. He wrote this article for the Chicago Tribune.