The Earth is warming. Here are the top warning signs, according to experts

The amount of greenhouse gases being emitted into Earth's atmosphere has reached such a high level that it will take major changes around the world to mitigate the effects on climate change, experts say.

Greenhouse gasses such as carbon dioxide, which trap the sun's heat, are the "most significant driver of observed climate change since the mid-20th century," according to the U.S. Environmental Protection Agency.

To illustrate the pace of change, the National Oceanographic and Atmospheric Administration's greenhouse gas index, which measures the impact of the gasses on climate, indicates it took approximately 240 years to go from 0 to 1, based on a 1990 benchmark. In the less than three decades since, the index says it has risen another 43% above the baseline.

"Climate change is now affecting every country on every continent. It is disrupting national economies and affecting lives, costing people, communities and countries dearly today and even more tomorrow," the UN says on its sustainable development site. "Weather patterns are changing, sea levels are rising, weather events are becoming more extreme and greenhouse gas emissions are now at their highest levels in history."

Here are the top issues contributing to climate change, according to the experts:

Contributing factors

Extraction of fossil fuels

The extraction of fossil fuels is one of the top issues affecting climate change, said Elizabeth Yeampierre, executive director of UPROSE, a New York-based organization that promotes sustainability, and co-chair of the Climate Justice Alliance, a network supporting the climate justice movement.

"We're literally going deeper and taking out fossil fuel when the science is telling us that we have to stop," Yeampierre said.

Fossils fuels can't continue to be burned at the current rate "if we want to have a stable climate," said Lindsey Allen, executive director of the nonprofit Rainforest Action Network.

"We immediately need to stop expanding our extraction of coal, and oil and gas," she said.

Burning fossil fuels emits large amounts of carbon dioxide and extraction generates methane, both of which are more abundant in the earth's atmosphere than they have been in 800,000 years, according to the EPA.

Nitrous oxide, the third principal greenhouse gas, which is also produced by burning fuels, has increased 20 percent since the start off the Industrial Revolution with the fastest rise in 22,000 years over the last century, the EPA said.

Transportation

The transportation sector generated the largest share of greenhouse gas emissions in the U.S. at 29% in 2017, according to the EPA.

The emissions primarily come from the burning of fossil fuels in cars, trucks, ships, trains and planes, and more than 90% of the fuel used for transportation is petroleum-based, according to the EPA.

Transportation accounts for nearly 20% of emissions worldwide, said Jason Smerdon, climate scientist at Columbia University's Lamont Doherty Earth Observatory.

Electrification of transportation, which would include electric cars, is one of the solutions to transforming the transformation sector, Smerdon said, as well as the development of hydrogen fuel cells and improvement of battery storage.

"We're in reach of what needs to be done to make all that happen," he said.

Electricity production

Electricity accounted for more than 27% of greenhouse gas emissions from the U.S. in 2017, and emissions from homes and businesses -- primarily from fossil fuels burned for heat -- accounted for another 11.6%, according to the EPA.

Nearly 63% of electricity in the U.S. comes from burning fossil fuels, namely coal and natural gas, according to the EPA.

Globally, more than 40% of emissions come from electricity and heat production, Smerdon said.

The energy sector presents the easiest possibility to transition to renewable, non-carbon based energy sources, which are "the cheapest form of energy out there," according to Smerdon.

While there is currently a "fairly rapid transition" to wind and solar energy, driven by economics, "the question is whether it's happening fast enough to reduce our overall greenhouse gas emissions," he said.

"We need to do a lot to accelerate that," Smerdon said.

Industry

All industrial sectors contribute to the emissions of greenhouse gases, Smerdon said. In the U.S., industry accounted for 22% of emissions in 2017, according to the EPA.

Direct emissions in this sector come from burning fuels or chemical reactions, the EPA said, and indirect emissions come from the power needed to run the plants and the fossil fuels burned for that.

The warning signs of climate change

The climate crisis is construed by some as a far-off event (or one that is not happening at all), but it's crucial to understand that the negative impacts are already happening, according to experts.

The earth is warming

Temperatures on Earth are already happening, Smerdon said.

As the surface temperature rises, the amount of ice and snow is decreasing, and the number and intensity of heat waves are increasing, Smerdon said.

With the increase of storage of energy within the earth's systems comes the increase in holding capacity of moisture in the atmosphere, he explained, which leads to bigger downpours, stronger hurricanes and unprecedented rain events.

"One-in-thousand-year events are happening every couple of years," Smerdon said.

"Temperatures are rising, snow and rainfall patterns are shifting, and more extreme climate events – like heavy rainstorms and record high temperatures – are already happening," the EPA says on its website. "Many of these observed changes are linked to the rising levels of carbon dioxide and other greenhouse gases in our atmosphere, caused by human activities."

Rainforests are burning at unprecedented levels

Setting fire to rainforests has been a method farmers have been using to clear land in Brazil for decades, but now because of drought conditions that have been affecting Brazil, they are burning hotter, longer and faster, Allen said.

Then, not only is a major source of carbon absorption and fresh water reservoir destroyed, but so is the biological diversity that resides within it, she said.

1 million species are at risk of extinction

Up to one million animal and plant species are being threatened with extinction due to human activity, some within decades, according to a United Nations report on the state of biodiversity and ecosystems published in May.

More than 40% of amphibians, nearly 33% of coral reefs and about a third of marine mammals are threatened, according to the report. An estimated 10% of the insect population is being threatened.

Food supply could dwindle

As droughts and downpours continue to impact agricultural areas, it will severely impact food supply, Smerdon said. Prices may also eventually be impacted, the EPA said.

Consumers will also feel those effects on their pocket books as jobs are lost and supply chains are disrupted, Smerdon said.

Currently, more than a third of the world's land surface and nearly 75% of freshwater resources are devoted to crop or livestock production, a UN report released Wednesday found.

Oceans are rising

The sea levels continue to rise as ice in the arctic continue to melt at a rapid pace, said Dr. Ayana Elizabeth Johnson, a marine biologist, environmental policy expert and founder of the Urban Ocean Lab, a think tank for urban coastal cities.

Oceans could rise by 1-2 feet by 2100, even if greenhouse gas emissions are sharply reduced, the UN report said.

Rising levels will then threaten coastal cities and islands with severe flooding. According to the EPA, the rate of flooding is increasing along areas of the East and Gulf Coasts.

Carbon dioxide levels have also risen in the oceans over the past couple of decades, according to the EPA, leading to an increase in acidity. Higher carbon dioxide levels have also led to a lower concentration of aragonite, which makes it more difficult for some marine animals to build their skeletons or shells, the EPA said.

Climate impacts are creating crises in the developing world

As the effects of climate change hits developing countries, it creates millions of climate refugees and exacerbates political instability, according to the experts.

The climate emergency is also a human rights issue, Allen said, and research shows a connection between higher climate temperatures to more conflict, Smerdon said.

This creates "complicated, cascading effects" of social structures and government structures, he said, and creates refugees as people leave their homes for safer climates, Johnson said.

"It is wrong to create hundreds of millions of climate refugees and then close our borders when they seek shelter on our shores," Johnson said.