World leaders meeting for a conference this week could set policies that would stop dangerous changes to the climate and keep ecosystems from collapsing further.

So far they have failed to do so.

The gap between what countries are doing to slow global warming and what they need to do to will hit about 28 gigatons of CO2 a year by 2030, according to a United Nations report published last Tuesday. The disparity is enough to warm the planet 2.7 degrees Celsius above pre-industrial temperatures by the end of the century.

That would break the promise world leaders made at the Paris climate conference in 2015 — in a bid to stave off unprecedented weather extremes like heat waves and cyclones — to limit warming to 1.5 C. They are instead burning so much coal, oil and gas that the world will likely cross that threshold as early as next decade.

To stand a chance of meeting the target, we have "eight years to make the plans, put in place the policies, implement them and ultimately deliver the cuts," wrote Inger Andersen, executive director of the UN Environment Programme (UNEP) that authored this week's report. "The clock is ticking loudly."

How can we meet the 1.5 C climate target?

The Intergovernmental Panel on Climate Change (IPCC) has shown that emissions need to fall about 45% from 2010 levels by 2030. By 2050, they would have to reach "net zero" — where people are adding as much CO2 to the atmosphere as plants and technology are sucking out.

This could be done while growing the economy and providing everybody with stable and affordable energy access, according to a roadmap of 400 milestones published by the International Energy Agency (IEA) in May.  However, achieving net-zero emissions by 2050 "will require nothing short of the complete transformation of the global energy system," the authors wrote.

Here's how.

Power 

Underpinning the transition is the shift from fossil fuels to renewable energy. Starting today, that would mean approving no new oil fields, gas fields or coal mines, according to the report. All coal and oil power plants whose emissions aren't directly captured would be phased out by 2040.

Instead, solar and wind would become the leading sources of electricity by 2030 — and together generate nearly 70% of electricity by 2050. Nuclear power would double by 2050 to mak up almost 10% of overall electricity generation.

Such a transformation would mean rich countries make all their electricity from low-carbon sources by 2035, with the rest of the world getting there five years later.

In India, the fourth-largest electricity market in the world, this would mean the end of a polluting coal industry. Across sub-Saharan Africa, where most people do not have access to electricity, it would mean avoiding investments in dirty infrastructure.

Building out renewable energy helps rural communities without access to electricity grids adapt, said Amos Wemanya, an energy analyst at Kenya-based think tank Power Shift Africa. The development goals of cutting emissions and becoming more resilient to increasingly extreme weather "go hand in hand," said Wemanya.

Transport

Even as some countries have managed to cut their total emissions — mainly by burning less coal — pollution from their transport sectors has continued to rise.

Bucking that trend would mean an end to the sale of new cars with internal combustion engines by 2035, according to the IEA. Heavy trucks would use biofuels in the short term before running on electricity and hydrogen after 2030. By the middle of the century, most vehicles would run on electricity or fuel cells.

While there are not yet climate-friendly alternatives to flying long distances, investments in rail and taxes on commercial flights could curb demand for aviation. In 2050, planes would rely on low-carbon biofuels and synthetic fuels to travel long distances without polluting the atmosphere.

Ships — among the only modes of transport that the report does not expect to decarbonize by the middle of the century — could burn less fuel if they were built and operated more efficiently. The use of ammonia as a fuel could power journeys across oceans.

Buildings

Demand for living space is set to grow 75% by 2050 and, as the planet heats up, the use of energy-hungry air-conditioning will also rise.

But emissions from buildings could decline 95% over the same time period.

This would be driven by electrification and gains in efficiency that use technologies already on the market: heat pumps, energy‐efficient appliances and buildings designed with materials that better regulate temperatures. Today, 1.5 million heat pumps are installed each month. According to the IEA roadmap, this would need to rise to 5 million by 2030, and 10 million by 2050.

By then, heat pumps would supply more half the global demand for heating. At the same time, fossil fuel heating would have to be phased out. From 2025, no fossil fuel boilers would be sold.

Industry

Hundreds of industrial plants would use hydrogen to make products like steel, or else use technology to capture carbon from any remaining fossil fuels they burn. These technologies, which provide the extremely hot heat needed in manufacturing, together contribute half the emissions cuts in heavy industry by 2050.

Blast furnaces and cement kilns, for instance, have an average lifetime of between 30 and 40 years. That means climate-friendly investments today would be needed to avoid having to tear down infrastructure early.

Different ways to cut emissions

While the IEA report provides a "detailed and useful roadmap... it is not the only way of doing it," said Joeri Rogelj, a climate scientist at the Centre for Environmental Policy at Imperial College London, UK.

Any plan for decarbonizing the economy, he added, requires key changes like rapidly scaling up renewable energy, phasing out unabated fossil fuels, stopping deforestation and cutting emissions from food. But scientists have different opinions on how much emphasis they place on nuclear energy, behavioral change and technologies to suck carbon out of the atmosphere.

The sooner emissions start to fall, the easier it will be to keep temperatures under control. Reaching the 1.5 C target isn't easy, said Amos Wemanya, but it's still possible. "It requires concrete, bold decisions."