A major international climate meeting is kicking off in Sharm el-Sheikh, Egypt. Hundreds of world leaders will spend the next two weeks discussing global efforts to dramatically reduce greenhouse gas emissions and pay for the costs of climate change.

The meeting comes at a crucial time for the planet: if nations, including the United States, follow through on their current promises to pivot away from fossil fuels, it's still possible to avoid catastrophic warming later this century. But it's unclear how, exactly, those promises will be met, and who will foot the bill for the deadly climate effects already underway.

Here's what you need to know about what's at stake and how the meeting will unfold.

1. Why is this meeting happening now?

The meeting that begins today is an annual event hosted by the branch of the United Nations that handles global negotiations about climate change. At the 2015 meeting, the Paris Climate agreement was signed.

Under that agreement, basically every country in the world promised to address climate change by coming up with their own plans to cut greenhouse gas emissions that are making the world hotter. Since then, the annual meeting has taken on extra significance, because it's the official time to check in on those promises, and make new ones. For example, the U.S. promised to cut its emissions by about half by 2030.

In United Nations jargon, the meeting is called the Conference of the Parties, or COP. This is the 27th Conference of the Parties meeting, so it's frequently referred to as COP27.

2. What has happened since last year's COP meeting?

At last year's meeting, world leaders agreed to transition away from fossil fuels and cut greenhouse gas emissions more quickly than in the past. But they failed to make substantive promises about how that would happen.

Since then, there have been some big geopolitical changes. The Russian invasion of Ukraine will loom over this year's meeting. The invasion further complicated relationships between the world's largest economies, and upended global fossil fuel markets. One immediate effect of the war is multiple countries including China have increased their short-term reliance on coal-fired power plants, which are the most intense global source of greenhouse gas emissions.

Another effect is that many countries, especially in Europe, are scrambling to develop new sources of natural gas to replace imports of Russian gas. In order to avoid the direst climate catastrophes in the future, nearly all new gas, coal and oil have to stay in the ground, experts say.

But there have been positive developments as well. Renewable energy, such as wind and solar, is growing rapidly. The International Energy Agency predicts that global demand for all types of fossil fuels will peak by the mid-2030s.

3. How much ground do world leaders need to make up at this conference?

In short, the world is way off track from its goal of cutting the pollution that drives climate change. Collectively, nations have promised to cut their emissions by about 3% by 2030. But the science shows emissions need to fall dramatically faster – 45% by 2030. That's to limit warming to the goal set by the Paris climate agreement: 1.5 degrees Celsius by the end of the century. That's about 2.7 degrees Fahrenheit.

Most nations aren't even cutting emissions enough to meet the pledges they've already made. So today, the world is heading toward about 5 degrees Fahrenheit of warming by 2100. While a handful of countries are expected to make new, more ambitious emissions pledges at these talks, the countries producing most of the climate pollution aren't expected to make dramatic cutbacks.

Two of the largest emitters, China and India, plan to increase emissions until 2030. They've argued that their growing economies need the support of fossil fuels, as other wealthier countries have historically done.

4. What do scientists say is at stake if world leaders don't make deeper cuts to greenhouse gas emissions?

The science is clear: the faster greenhouse emissions drop, the more lives and livelihoods will be saved. And the sooner, the better.

We know that because earlier this year, international climate scientists finished publishing the most comprehensive climate science report ever. It catalogued the ways in which climate change is affecting everyday lives around the world, because the Earth is already about 2 degrees Fahrenheit (1 degree Celsius) hotter than it was in the late 1800s.

Scientists say it's possible to limit overall global warming to about 3.5 degrees Fahrenheit, (2 degrees Celsius), which is the upper limit set by the 2015 Paris climate agreement.

Scientists also warn that decades of sea level rise, extreme drought, heat waves and storms are unavoidable because of how much global temperatures have already risen. That means billions of people will need to adapt to a hotter Earth.

But limiting emissions could avoid some of the most extreme impacts, like much more deadly heat waves, more flooding in coastal cities due to sea level rise and the loss of almost all coral reefs.

5. What will be the most contentious topics at these talks?

Developing countries are getting increasingly frustrated with wealthier nations. Most low-income countries have done little to cause climate change, since their greenhouse gas emissions are small. But impacts like extreme storms and flooding are taking a huge toll on them, often because they lack the resources to better protect themselves. This year, a coalition of the most vulnerable countries is seeking compensation for these costs, known as "loss and damage."

They argue that wealthier nations should pay for the problems they caused, including the cultural losses that happen when towns and villages must relocate. So far, wealthier countries have agreed to keep discussing it, but haven't committed to providing new funding.

6. How much would it cost to deal with climate change?

It's going to require huge investments. There's no getting around it. But there's also a lot of money to be made eliminating emissions from the global economy. And experts say the cost of not dealing with this problem could be ruinous.

In the United States alone, quickly cutting carbon emissions could grow the country's economy by $3 trillion over the next 50 years, says Deloitte, the consulting firm. On the other hand, not doing enough to respond to climate change could cost the U.S. $14.5 trillion over the same period.

At COP27, one of the biggest issues is going to be money that developed countries promised poorer nations years ago to help them cut emissions and adapt to the climate impacts they're already experiencing. Since industrialized nations are responsible for most of the emissions that are making the planet hotter, they pledged $100 billion a year in financing for developing countries by 2020. But rich countries still haven't delivered.

Experts say making good on that promise is crucial to keep poorer nations on board with efforts to cut emissions. But they also say that $100 billion is just a fraction of the money the developing world is going to need.