A friend, who is an intelligent person with no science background, asked me to explain simply what the concern expressed with alarm by many scientists and (anti) climate change activists is about the increasing rate of methane gas emissions in the Arctic. That attempted explanation follows.
From even before the extinction of the dinosaurs by the Chicxulub Meteor 66 million years ago (66mya), to about 34mya, the Earth was much warmer (the peak occurred 50mya) and there was no polar ice, north or south.
Antarctica was covered in forests and jungles; the Arctic Ocean was a warm sea ringed by swamps and forests of ferns and Redwood trees along the Eurasian and North American northern continental shores; and those swamps swarmed with crocodiles.
Between 34mya to 12mya Earth’s temperature fluctuated and Antarctica froze thawed and refroze. Then Panama swung into place closing the oceanic gap between North (Central) and South America, and that altered ocean currents so that a Southern Ocean circumpolar current sealed off Antarctica climatically: the deep freeze of that continent that continues to this day.
That global cooling trend continued after 12mya and plunged Earth into the deep cold of the repeated glaciations of the Pleistocene Epoch (Ice Ages), from 2.58mya to 11,700ya, before the thawing of temperate latitudes introduced the balmy global climate we have enjoyed since.
All the lush and soggy vegetation around the Arctic Ocean was buried by sedimentation into the shallow continental shelves around that ocean, and then further locked away by the deep freeze producing permafrost, which extends quite a bit down below the ground surface, and down from the top of the seafloor of the shallows near land.
Rotting organic matter in the seas (algae, plants, fish, animals) sinks to the bottom and is decomposed by bacteria, and that produces methane gas (like cows fart from eating grass, and we fart from eating beans); but because of the cold and pressure deep down in all oceans, or in cold shallower seas like the Arctic, that gas actually combines with water into a fragile unstable crystal-like solid called methane clathrates or methane hydrates.
This is an “ice” that people can light up with a match and it burns like gas-soaked charcoal, but with a blue flame. When a methane hydrate solid is brought up to the surface of the ocean from the high pressure of the depths, it can spontaneously ignite because of the release of methane gas mixing with the oxygen in the air. Such flares have been seen on the ocean surface at night by airline pilots.
There is a large amount of compressed, frozen methane-rich organic matter, including peat, all along the sub-Arctic ring of sea and land about the Arctic Ocean. The thawing of that region is now increasingly releasing some of the trapped gas: from out of the clathrates, from out of subsurface compressed organic plant matter, and also from new underground fires burning peat seams and coal seams. Such fires are now extensive and burning continuously all along northern Siberia; they are called Zombie Fires.
Because of the complexities of molecular structure, a molecule of methane (CH4) has 2.5x (15/6) more ways of moving, plus rotating about and vibrating along the chemical bonds between its atoms, so as to store heat, than does a molecule of carbon dioxide (CO2). So, CH4 is 2.5x times more effective at being a global warming agent than CO2.
A large release of CH4 into the atmosphere will have a more pronounced global warming effect than an equal mass of CO2. But CH4 eventually combines with atmospheric oxygen molecules to form more CO2 and H2O (water).
What is happening in the Arctic is that the massive amount of stored subsurface methane — in all the forms that bound it — is now being warmed sufficiently to allow it to overcome the cold and pressure that used to hold it in. So there is an increasing rate of methane gas bubbling up from the seafloor, and from the Arctic tundra which is permafrost grassland that is thawing, slumping, and popping out with methane eruption craters, some tens of meters in diameter and depth. , 
Because of that accelerating rate of emission, and because the total amount of methane stored in the Arctic is so large, climate scientists are very concerned about the negative potential for our climate in the near future.
How worried? How fast? How alarming?
Well, the presently accelerating rate of carbon dioxide buildup in the atmosphere, and of global warming, is proceeding at a pace at least 20x that of previous major CO2 eruptions and global warming events in Earth’s geological past (like during the onset of the Paleocene-Eocene Thermal Maximum, 55.5mya); and that rate today could even be hundreds of times faster.
The CO2 increase in the atmosphere over the last century or so has equaled comparable amounts of increase that may have occurred over several thousand years during the massive eruption episodes in the geologic past that caused major extinctions.
During those past eruption events, where the pace of change was over thousands of years (a blink of the eye geologically), despite the extinctions that occurred much animal and plant life was able to adapt, and such adaptation carried on over longer spans of time was their transformation by biological evolution.
But today such a tactic of biological adaptation by a species in response to the shifting of climates is impossible because the genetic processes of evolution are far outpaced by the rapid rate of increase of CO2 concentration, and thus of global climate change.
However, we are not talking about doomsday in 5 or 10 years. Just think of how climate and weather have changed (gotten worse) since, say, the 1970s, and imagine a similar rate of degradation for another few decades, and you can then guess that sometime near the end of this century (maybe the 2070s) that Earth will really be at the edge of environmental collapse: if humanity had continue to do nothing about curbing its greenhouse gas emissions since this moment, and continues heedlessly emitting fossil fuel exhaust fumes beyond that point.
Many people worry that such an unhappy timetable could be sped up if there were to be a truly massive eruption of “all” the methane locked up in the Arctic. If I get to live to be 100, in 2050, I’ll then know the ultimate course of Earth’s dynamic climate system.
Young people worldwide, sparked by Greta Thunberg , will be alive in 2050 and very much want to know NOW what the environmental conditions will be THEN, when they are supposed to experience their adult lives and be responsible for continuing civilization. And they have every right to demand that today’s adults do their intergenerational duty to pass on a hospitable Earth that sustains their dreams, our human civilization, and all species’s futures.
Within the next 10 years we had better begin to actually and continually cut down civilization’s (anthropogenic) annual CO2 emissions; by 25 years we had better be reducing them at a very pronounced rate; otherwise by 50 years Earth’s temperature may be high enough to trip the climate system into a new mode we will very much dislike — being much more of what we don’t like now — and which will be beyond our ability to correct regardless of whatever heroic measures we would then take, like miraculously dropping our CO2 emissions to zero forever.
The geophysical reality is that it takes the climate system hundreds of years (I once estimated 240 years) to BEGIN to shift in response to new atmospheric conditions. This is like a huge thermostat lag to a heating system of global scale, or like the lag between turning the rudder on a large ship and then actually having the ship begin to veer in a new direction.
It is because of this inertia that it is essential to stop our emissions as soon as possible (ASAP). The longer we wait — emitting more while waiting — the longer it will take Earth to respond to our finally throttling our emissions, and the longer it will take for the climate system to flush out that excess CO2 and lower the average global temperature. I estimate 1,000 to 1,400 years, but it could be much longer.
So that is what the worry about the increasing Arctic methane releases is all about.
 “I Am Greta,” an excellent documentary about the young lady who is puncturing the big phonies of all our governments, on the overarching issue of climate change.