“A watched pot never boils,” the saying goes. We humans find it hard to register very slow change, even when it produces tangible impacts. We don’t notice the day-to-day growth of our kids, even though they repeatedly outgrow pricey clothes for us to replace.

When we say something proceeds “at a glacial pace,” we mean it’s extremely slow. Slower than a snail’s pace.

Soon the term will carry a different connotation. Around the globe, once-slow glaciers are speeding to their own destruction, and hastening ours.

Most of us, caught up in the eternal now, remain oblivious to global temperature and sea level rise. Now and then we “buy new clothes,” contending with extreme events like Sandy and Katrina at great human and fiscal cost. After the shock value fades, we’re back to issues that matter, like Sunday sales.

The glaciers’ demise is already raising sea levels, threatening coastal and island communities in real time. Glaciers provide crucial sources of potable water. They’re primary contributors to the vast “conveyor belts” that circulate air and water. They nourish and shelter important ecosystems.

Huge ice sheets in Greenland and the Antarctic are fragmenting, prone to breaking up and floating away. These don’t always influence sea levels like melting glaciers do. Many ice sheets are actually floating, supported by the water beneath them.

When floating ice melts, sea levels aren’t affected. Drop an ice cube in a glass of water and mark the water level. Measure again after the ice melts. Doesn’t change. Just physics.

However, some sea ice rests on the shallow sea floor beneath it. Although surrounded by water, it isn’t floating. When it melts, it adds its volume to that of the sea. So it is with melting glaciers.

Early this century, as it became apparent that ice sheets can respond to climate change in mere months and years, glaciology research surged. We can measure ice on scales ranging from inches to continents.

Glacial field research, conducted up close and personal, has now been augmented by many other technologies. Ground-based weather stations, seismometers, time-lapse cameras and new radar reveal elusive new data. Aerial surveys allow canvassing areas too dangerous for pedestrians, places riddled with crevasses and sinkholes.

Since some endangered ice spans vast distances, space-based data have become essential. Satellites can measure minor fluctuations in gravity to estimate ice-volume variations. Altimetry satellites assess changing surface elevations. Optical and radar sats measure motion, the advance and retreat of ice masses. They observe surface changes too as ice melts, or blackens from deposits of airborne soot. (Sooty ice absorbs more solar energy, speeds melting and prevents some heat from escaping back into space.)

We’re finally starting to accumulate the long-term data needed to separate the effects of glacier “climate” from glacier “weather.” This immense body of detailed data clearly points to rapid and largely irreversible ice loss.

Climate denialists seem to believe that if climate change does exist, and if it does cause problems, then human ingenuity will set matters right. It doesn’t work that way. Changes underway right now will increase and persist for centuries; even if we muster the political will to conduct radical carbon-emission interventions, we won’t be able to close this barn door until long after the horses swim away. Diamonds and ice loss are both “forever,” from a human perspective.

What are we losing? It’s most visible in worldwide glacier retreat. Consistent, severe and unprecedented glacial shrinkage affects the Antarctic Peninsula, Patagonia, mountains like Kilimanjaro and the Himalaya range, the immense ice reservoirs of Greenland and the Arctic. Montana’s Glacier National Park has lost many of its iconic glaciers in our lifetime. The loss is visibly speeding up.

A few glaciers defy the trend toward retreat, but this is largely due to cyclical interactions between glaciers and sediments, independent of climate; or climate-driven changes such as increased local precipitation.

The West Antarctic Ice Sheet rests on bedrock, well below sea level and exposed to warmer deep waters. If, or when, it melts, this sheet will add 5 meters to the sea level. That’s about 16 feet. Multiple studies indicate this West Antarctic collapse is already underway, and could well be irreversible even now. So long, Miami. New York. New Orleans. Tourism will require SCUBA gear.

Glaciers cover about 10 percent of the Earth’s surface, and store about 75 percent of the world’s fresh water. Their disappearance will be disastrous for those who rely on rivers fed by seasonal glacial run-off. These include rivers that derive part of their flow from seasonal monsoons, or annual snowpack melting. Upstream populations, closer to their normal glacial sources, will be affected more than those who live downstream. But even when torrential rainfall augments flow, it’s seasonal and unreliable.

To cite one example: Many Asian rivers rely on Himalayan glaciers to provide drinking water, irrigation and power generation for 1.5 billion people in nine countries. Some of these already go dry seasonally.

Reduced glacial flow will compromise human prosperity, if not survival, along the Ganges, Indus, Mekong, Yangtze, Yellow, Brahmaputra, Irrawaddy and Salween rivers.

The evidence is overwhelming: The Earth is losing its ice. Much of this loss is irreversible, and results from human-caused climate change. Useful action to slow or arrest approaching calamity is possible, and feasible.

We need the political will, but we’re ruled and bamboozled by a cadre of political Won’ts.

Jon Hauxwell, MD, is a retired family physician who grew up in Stockton and lives outside Hays.