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After the Maui Writers Conference last week, I took advantage of the glorious weather and hiked through Haleakala Crater. I smeared large dollops of sunscreen on my skin before I started walking, but with no clouds in the sky, and an elevation of 10,000 feet, I still worried about the sun on my face.

A hat would help, but the only one handy was a neoprene visor my husband Craig had worn while windsurfing the day before. I pulled the still-wet hat low over my forehead, and we started walking. "Do you smell something?" I asked Craig as we headed down Sliding Sands Trail.

"Like what?"

"Like dead, rotting animal."

He sniffed the air. "I don't smell anything."

We walked on, but the smell, vaguely familiar, did not diminish. Then, in a flash of clarity, I knew what the odor was and where it was coming from. I pulled the visor off my head, sniffed it and held it to Craig's face. "What does that smell like?" I asked.

He wrinkled his nose. "Like dead, rotting marine animal. I must have fallen in some plankton or something."

When we get a whiff of the ocean, most of us say, "Ah, smell that salt air." But we know from our home salt shakers that salt doesn't have a scent. We can taste the salt on our lips after a walk on the beach, but salt isn't what we smell.

What is it then that we are detecting when that romantic smell of the sea drifts to our nostrils? Well, it's anything but romantic: It's a gas called dimethyl sulfide.

It is a naturally occurring gas that microscopic marine plants give off when microscopic marine animals eat them. Because there are enormous numbers of these plants and animals in the oceans, there's a lot of dimethyl sulfide in the air. Researchers estimate that the world's oceans give off about 60 million tons of the gas each year, which break down to about 30 million tons of airborne sulfur.

This atmospheric sulfur plays an important role in controlling the oceans' surface temperatures, and therefore Earth's temperatures. Since sulfur molecules reflect radiation from the sun, they turn back into space some of the rays that would otherwise strike the oceans.

Sulfur from dimethyl sulfide also makes clouds over the oceans denser. Dense clouds reflect the sun's rays more than thin clouds and so also prevent some of the sun's rays from reaching the oceans.

With less sun the oceans get cooler, their plant growth decreases, and the release of dimethyl sulfide goes down. That soon decreases the amount of sulfur in the air, causing the oceans to warm up and grow more plants. This cycle regulates ocean temperatures and, subsequently, temperatures of the entire earth.

Erupting volcanoes also release sulfur compounds that affect global temperatures. Our Hawaii volcanoes don't alter the weather much, but when the Philippines' Mount Pinatubo erupted in 1991, the sulfur gases (and other airborne particles) caused measurable global cooling in 1992.

Then there's industrially produced sulfur, which does the same as ocean-produced sulfur: It reflects the sun's rays. So when our factories spew sulfur into the air, there's more to worry about than acid rain.

The presence of all these sources of fluctuating dimethyl sulfide in the world is one of the reasons it's difficult for researchers to accurately predict changes in Earth's climate.

Craig's stinky visor soon dried out during my walk in the hot crater, and the smell mercifully vanished. I like the smell of the ocean, but a little dimethyl sulfide goes a long way.