Climate science—like all science—is based on collecting, analyzing, and interpreting data.  Scientists around the world share data to get an accurate picture of how the complex system of Earth’s climate works. They track trends and investigate anomalies. This data also tells them what the climate was like in the past and how we can expect it to behave in the future.

But how do they do it? What are the crucial measurements, events, and indicators? And how do we know what’s causing the current global warming trend?

What we call climate refers to the average weather over a period of time in a given area. Climate means, for example, the average temperature in Utah in spring. It’s also how many hurricanes happen, on average, in the Gulf Coast in summer. If spring in Utah is hotter than it was a generation ago, or there are more hurricanes in the Gulf Coast than in past decades, it’s a sign that the climate is changing.

Weather, on the other hand, is what happens day-to-day. A summer snowstorm is unusual weather—a pattern of late-season snowstorms might signal a changing climate.

The Earth’s climate does shift naturally—but natural changes happen much more slowly than what we’ve seen in recent decades. Scientists have detailed data from ice cores going back 800,000 years that shows them how natural climate change works. Climate scientists have used the same data and methodologies to determine that the current warming trend can only be caused by human activity. We know what the climate should be like at this point in the Earth’s climate cycles based on data on natural climate change factors. We know that it is far warmer than it should be. And it’s only getting warmer.

Natural causes of climate change

Many factors influence the Earth’s climate—many of them are natural phenomena. But none of these factors come close to explaining the warming patterns that scientists have observed over the last two centuries.

Variations in greenhouse gases

Carbon dioxide and methane levels in the atmosphere have a direct relationship to global temperature. Both of these greenhouse gases act like thermal blankets by reducing how much heat dissipates from Earth’s surface into space. Although carbon dioxide and methane occur in only trace amounts in the atmosphere, they’re a key indicator of global climate: variations in their concentrations have coincided with past warm and cold periods. Natural variations in these greenhouse gases generally occur very slowly and have remained within a predictable range during the past hundreds of thousands of years—until the Industrial Revolution.

One way scientists can measure past carbon dioxide levels is by analyzing samples of ice from Antarctica or Greenland. Glacial ice traps air bubbles, then gets buried by more snow and ice—so samples taken from deep below the surface contain ancient air bubbles preserved thousands or even hundreds of thousands of years ago. The lowest atmospheric concentrations of carbon dioxide coincide with glacial periods, or ice ages. The highest levels of carbon dioxide correspond with the warm periods between ice ages. These are called interglacial climates; we’re in one now. For about the past million years—even during interglacial periods—carbon dioxide concentration was never more than 300 parts per million (ppm). Today, it’s above 400 ppm.

Variations in Earth’s orbit

Variations in the Earth’s orbit can cause massive climate change—over the course of thousands of years. These shifts are called Milankovitch cycles. There are three variations in the Earth’s orbit: how the Earth wobbles on its axis as it orbits, the degree of the Earth’s tilt toward the sun, and the shape of the Earth’s elliptical orbit around the Sun. These three cycles happen simultaneously but on very different timescales, which can produce a combined climatic influence that affects, among other things, the timing of the ice ages. Further proof that the Milankovitch cycles don’t explain our current warming climate: their current situation indicates a cold climate—possibly even the beginning of an ice age.

Variations in solar energy

The Sun’s energy fluctuates over decades and centuries based on how many dark spots (sunspots) are on its surface. The sunspots appear and fade on an 11-year cycle that is thought to have a small effect on climate. But larger variations can take place over centuries, and help explain the “Little Ice Age” the Earth experienced a few hundred years ago when the Northern Hemisphere experienced unusually cold weather.

Volcanic eruptions

Volcanoes can either warm or cool the climate—it all depends on the timespan. Major volcanic eruptions actually have a short-term cooling effect. Clouds of ash from eruptions darken the skies over the surrounding area. Volcanoes also shoot sulfur dioxide gas into the stratosphere, which combines with water vapor to form sulfuric acid droplets that block sunlight and cause a temporary global cooling that typically lasts two to three years. But volcanoes also warm the climate by releasing greenhouse gases. Millions of years ago, when volcanoes were extremely active, this may have caused very warm global climates. It’s doubtful that volcanoes will significantly affect global temperature trends in the next century.

The climate does change naturally. But It’s important to distinguish between the types of changes that take place naturally, over long time periods, and the accelerated changes we’ve seen—and expect to keep seeing—since the Industrial Revolution. The scientific consensus is that none of these forces explain our current era of climate change. How do we know? Scientists diligently collect data and use it to reach a better understanding of our planet’s climate.

Scientists have been collecting data about the climate—temperatures, rain and snowfall, and atmospheric gases—for centuries. This data is used in climate models: mathematical algorithms that simulate future climate.

Weather measurements from sources around the world can be assimilated into climate models to produce a comprehensive, three-dimensional representation of global weather conditions. Climate models that are used for future prediction are carefully tested to ensure accuracy. One way scientists do this is to hindcast the recent past: they feed the climate model a limited data set of inputs and then compare the output results to real, observed data.

Scientists focus on three main variables when studying the climate: temperature, precipitation, and carbon gases.

Temperature

Scientists take Earth’s temperature in two ways: records of the temperature taken from the Earth’s surface and measurements by satellites. Thousands of weather stations—collectively, the Global Historical Climatology Network (GHCN)—cover land regions around the world, recording local temperatures. Some of these stations have been monitoring the climate for over 150 years. Ships and buoys measure the ocean temperature, and that data is combined with the land-based thermometers for an accurate picture of the Earth’s average surface temperature. Scientists can use these data sets to study variations in climate over time.

Satellites also measure the Earth’s temperature and can cover areas where there are no surface thermometers. They also provide readings of every level of Earth’s atmosphere. While their reach is superior, satellite readings are less accurate than surface measurements, and their data only goes back a few decades.

Rain and Snow

The best way to measure precipitation is to collect it in a gauge and measure it directly. Data from tens of thousands of gauges, spread out over every continent, gives us a picture of precipitation on Earth.

Gauges may be the most accurate, but they only work on land—not across the oceans. For that—again—we turn to satellites. Satellites can measure precipitation across the surface of the entire planet. A new satellite mission called the Global Precipitation Measurement observes rainfall and snowfall around the world every three hours. Satellites give us the widest picture of precipitation, but their records don’t go as far back as land-based gauges, and they’re not as accurate.

Carbon Gases: Carbon Dioxide and Methane

Carbon dioxide and methane play crucial roles in climate dynamics. Carbon dioxide levels don’t vary much around the world because the gas travels and mixes in the atmosphere. Since 1958, the Global Greenhouse Gas Reference Network maintained by the National Oceanic and Atmospheric Administration (NOAA) has been measuring carbon dioxide at Mauna Loa Observatory in Hawaii. Before that, measurements of carbon dioxide were haphazard. According to measurements at Mauna Loa, carbon dioxide has increased by more than 25% since the 1950s.

Global methane measurements began in 1983. The global methane concentration is also well mixed in the atmosphere, but it varies more from one location to another than carbon dioxide.  Since we began measuring it, methane has risen by more than 10%.

Clues to past climates

To understand contemporary climate change, it is important to know how and why climate has changed in the past. There are many clues—or “proxy records”—that scientists use to understand how temperature and precipitation behaved in the past, even before thermometers and rain gauges existed.

Historical Records

For the past few hundred years, historical documents are valuable sources. Shipping logs, agricultural records, and newspaper accounts of exceptional weather are good sources of climate data. Records of lakes forming ice and melting are particularly useful—they help us identify warming trends that started before the global thermometer network.

Vegetation Records

Plants are excellent indicators of past climates. Unlike written records, plants can tell us about the climate tens of thousands of years ago. Most plants only flourish in one particular climate. If traces of pollen from a cold-climate tree—like spruce—are found in an area that now supports warmer-climate trees—like oak—it’s an indication that the area used to be colder. The trees themselves might also give us important clues. If the annual growth rings of a tree vary from year-to-year over a period of time, scientists know that local temperature or rainfall was fluctuating.

Ice Sheets

Air bubbles trapped in glaciers and ice sheets provide another natural climate record. Every year, snow falls on existing ice sheets and glaciers, and every year, it doesn’t entirely melt. This cycle gives us a record of each year’s precipitation that goes back hundreds of thousands of years. Pockets of air within the snowfall are compressed into tiny air bubbles that retain the chemical makeup of the atmosphere as it was when that snow fell. These air bubbles can tell scientists about the local temperature, precipitation, and greenhouse gas concentrations. Climate scientists harvest this precious frozen record—which goes back 800,000 years—in a process called ice coring, usually in Greenland and Antarctica.

Trust the data

Very little in the scientific world is considered to be a rigid fact: the scientific method is based on hypothesis, and hypotheses should continuously be tested with new information. Climate scientists (much like astrophysicists or geneticists or paleontologists) don’t claim that they know the full and complete truth about our climate, let alone about our future climate. But the evidence overwhelmingly shows that human activity has caused—and continues to cause—rapid climate change. Scientists will continue to gather data and learn more about the situation. As for the rest of us, we must make our decisions based on the best information available to us. And that information tells us we need to act.