Historians and climate scientists use a variety of tools to measure climate in the past including tree-rings, ice cores, ocean and lake sediment, historical accounts, agricultural production documents, and economic indicators such as tax and tithe records. Economic historians have traced grain and wine prices in Europe back for centuries. In times of scarcity supplies were low and hoarding drove up prices. Ship captains and shipping companies, concerned about icebergs and frozen rivers and ports kept detailed records of icing. The most detailed and precise climate histories come from ice core drilling in Greenland and Antarctica. They record changes in climate over the last 800,000 years.
We usually think of sponges as airy water absorbent pads made of plastic or cellulose that we use for cleaning kitchen surfaces, washing dishes, or picking up spilled liquids. Natural sponges still used in bathing are from previously living things, the external skeletons of humble sea creatures in the species Spongia officinalis that live in relatively shallow seas in well-oxygenated water. Sponges can live for hundreds and even thousands of years, which makes them invaluable for tracking the Earth’s climate history. Three-hundred-year-old grapefruit-sized sclerosponges from the Caribbean Sea predate the Industrial Revolution and their skeletons contain carbon and oxygen isotopes in different ratios that provide evidence for global warming and its impact on the world’s oceans. Changes in isotope rations show that warming is occurring much faster than scientists initially thought, which means human civilization may be getting closer to a tipping point where catastrophic climate events can no longer be prevented or even moderated.
For those interested in chemistry, oxygen atoms contain either 8 (O16) or 10 (O18) neutrons. In a warmer climate, ocean water contains a higher percentage of oxygen with 8 neutrons. When the oceans cool, there is a relatively higher percentage of oxygen with 10 neutrons. The oxygen bonds with carbon and calcium to form calcium carbonate (CaCO₃) in the exoskeletons of sclerosponge.
Chemical variations in the spine of Caribbean sclerosponges show that between 1700 and 1790, ocean temperatures were stable. Between 1790 and 1860, there was some cooling, probably because of volcanic eruptions that blocked sunlight and created a worldwide cooler planet. These included major eruptions in modern day Indonesia and periodic, but smaller, eruptions of Mount St. Helens in Oregon. In the mid-1860s, the Caribbean Sea began to grow warmer. By the middle of the 20th century, sponge records suggest that the amount of global warming caused by the burning of fossil fuels was actually about half a degree Celsius (about 1°F) greater than scientists estimated using conventional methods.
Warming oceans have major climate repercussions. Hurricane winds become more powerful with a tendency to travel slower and stall over land longer causing more damage. Clouds carry more water, so storms unleash more rain, producing more flooding.