If the world waits until 2020 to take action on global climate change, it will undoubtedly be too late, the International Energy Agency (IEA) warns in a new report.

However, the Paris-based agency, which is tasked with maintaining global energy security, identified four proven policies, each relying on current technology, that could be implemented immediately. (See quiz: "What You Don't Know About World Energy.")

These efforts could keep the world on track while nations work toward a more comprehensive 2020 agreement to limit the rise in global temperature to no more than 2°C above preindustrial levels, IEA said. Without such steps, the agency said, the prognosis is dire. (See related story: "IEA Outlook: Time Running Out on Climate Change.")

"The path we are currently on is more likely to result in a temperature increase of between 3.6 degrees Celsius to 5.3 degrees Celsius," with most of that warming happening within this century, said IEA Executive Director Maria van der Hoeven at a London press conference. Already, the report noted, global temperatures have increased 0.8°C beyond preindustrial levels.

The IEA said that, under its four-point plan, greenhouse-gas emissions in 2020 would be 8 percent, or 3.1 metric gigatons, lower than levels they are otherwise likely to hit. In May, it was reported that the carbon dioxide (CO2) concentration in the atmosphere at the Mauna Loa Observatory in Hawaii hit 400 parts per million, a high in human history, and the IEA's goal is to help ensure that the level does not exceed 450 ppm by 2020. (See related story: "Climate Milestone: Earth's CO2 Level Passes 400 ppm.") But even if that goal is met, it said, there is still only a 50 percent chance of keeping to the 2°C limit.

IEA's four recommendations are:

Strengthen or introduce energy-efficiency measures in buildings, industry, and transportation.

Cut back on the construction and use of inefficient coal-fired power plants.

Take action to halve the release of methane into the environment from the oil and gas industry.

Start the phaseout of fossil-fuel consumption subsidies.

Here's a closer look at why IEA issued its new warning and suggested plan of action.

Q: Why is it important not to exceed the 2°C limit?

A: Even at the 2°C level, the Earth is likely to experience "extreme changes in its climate system that it has not seen in more than 200,000 years," said Corinne Le Quere, director of the Tyndall Center for Climate Change Research at East Anglia University. There will, she said, be "more heat waves, extreme rainfall events and rising sea levels." Severe weather, like the storms that caused the Danube and Elbe rivers in Central Europe to overflow in recent weeks, is expected to become more frequent. (See related: "Pictures: Worst Floods in European History?") Climate change will also place stresses on food production. Temperature increases greater than 2°C will be much harder to adapt to, Le Quere said, because there will be complete shifts in vegetation and water patterns.

Q: What's the most important step we can take now?

A: Of IEA's four recommendations, ramping up energy-conservation measures would have the greatest impact. Fully half of the 8 percent reduction in emissions would come from making residential and commercial buildings, industrial motors, and vehicles more energy-efficient, the report said. Policies to encourage energy conservation already exist in the United States, Europe, Japan, and China, though they need to be strengthened or extended, IEA said. (See related: "Pictures: Seven Supergreen U.S. Government Buildings.")

The agency is urging other countries to introduce similar measures to promote greater use of efficient heating and cooling systems, automated climate control systems, better insulated buildings, low-energy motors, and energy-efficient lighting and appliances. (See related: "'The Greenest Home: A Window on 18 Super-Eco Dream Houses.") Lights and appliances alone account for 37 percent of electricity demand in the mostly wealthy countries that are part of the Organisation for Economic Co-operation and Development (OECD).

Q: So why aren't we more energy-efficient?

A: Conservation is perhaps the "least painful" way of cutting emissions, as well as one of the best, said Le Quere. Nevertheless, she added, "one of the issues, one reason it's not taken up more is the cost, because you pay more up front." (See related: "Pro-Environment Light Bulb Labeling Turns Off Conservatives, Study Finds") Ultimately, those extra dollars are returned in the form of lower fuel bills, but "consumers don't see the gains right away," she said. IEA also urged tighter fuel-economy standards for vehicles—road transportation is the source for 16 percent of CO2 emissions from the energy sector. But the agency acknowledges that new rules won't have much immediate impact because of the long lead time needed to put newer, cleaner cars on the road.

Q: Why go after coal-fired plants?

A: About 20 percent of potential emissions abatement could come from cutting back on coal-fired plants, the IEA said. It could be part of a scenario that boosts the share of global power generation from renewable sources from 20 percent to 27 percent by 2020. IEA urges an outright ban on new construction of subcritical (inefficient) coal-fired plants (while allowing those already under construction to move forward). The report also supports idling aging coal plants that have recouped their investments, resulting in a worldwide 25 percent reduction of those facilities by 2020.

Q: Why focus on methane?

A: Methane is a potent greenhouse gas, with 25 times the global warming potential of CO2. (See related: "Methane: Good Gas, Bad Gas.") In 2010, uncontrolled methane released into the atmosphere totaled 1.1 metric gigatons of CO2 equivalent, or twice the total natural-gas production of Nigeria. The IEA reckons that 18 percent of the overall emissions reduction it's aiming for could come from cutting leakage in half. The agency considers this a rather easy policy to enact because the necessary technologies to capture methane emissions are readily available at low cost. Several countries—including the United States—are already adopting policies to require them. (See related:"A Move to Capture 'Fugitive' Natural Gas Emissions.") Paul Stevens, a senior energy fellow at London's Royal Institute for International Affairs, agreed. More often than not, he said, "methane emissions result from poor well completion." But for that problem to be fixed, he said, "you need to get regulators to do their jobs." And in many developing countries, that's not always easy to do. But a regulatory crackdown on methane emissions, he added, would also benefit the industry because it's profitable to capture and sell the gas instead of wasting it.

Q: Why are fossil-fuel subsidies a big deal?

A: The agency says 12 percent of the potential emissions reduction could come from scaling back subsidies. (See related: Interactive Map: Fossil Fuel Burden on State Coffers) In 2011, it said, fossil-fuel subsidies totaled $523 billion, or six times the amount of support given to renewable fuels: "Currently, 15 percent of global CO2 emissions receive an incentive of $110 per tonne [metric ton] in the form of fossil-fuel subsidies, while only 8 percent are subject to carbon price." Stevens also thinks this is a workable idea. "Pushing prices up is always a good way to convince people to use less." The IEA, however, admitted that "subsidy reform is likely to be a challenging and slow process in many countries because of political obstacles," which is why it doesn't envision a universal phase out any time soon. (See related: "Pictures: Eleven Nations With Large Fossil-Fuel Subsidies.") In January 2012, for instance, the government of Nigeria announced the complete removal of subsidies, which sparked weeks of violent protests. (See related: "Nigeria's Rocky Effort to Wean Itself From Subsidized Fuel.") "That was a classic example of how not to do it," Stevens said. But, he added, "it's not impossible" to remove them if there is political will and it's done slowly. He cited Iran as a country that successfully ended its subsidies. (See related: Quiz: What You Don't Know About Energy Subsidies.)

Q: Given the perilous state of some economies, won't these measures cost too much?

A: Not at all, the IEA said. Fatih Birol, the IEA chief economist and the report's lead author, said the recommendations are "proven measures that could stop the growth in global energy-related emissions by the end of this decade at no net economic cost." For instance, while the reduction in coal plants would result in $1.8 trillion in lost revenue through 2035, net revenues from renewables-based and nuclear power plants would rise by a similar amount. Moreover, IEA estimated that while delaying stronger action until 2020 might save industries $1.5 trillion in low-carbon investments, those industries would then have to spend $5 trillion through to 2035 to make up for the lost time.

In other words, when it comes to climate change, the cost of action pales beside the price of delay.