Human-induced climate change is happening. But in order to explore the evidence for this claim, we must first ask two questions: “What is science?” and “What is climate?” Once we understand how science is done and the basic dynamics of the climate system, we’ll focus on how scientists study ice cores, and see how evidence of climate change in the past is fundamental to understanding what lies ahead.

Both the ocean’s sheer size – it covers seventy percent of our planet’s surface – and the properties of water make the ocean a major player in Earth’s climate system. An enormous reservoir of heat, the ocean is an important mechanism of heat storage and exchange with the atmosphere, which has important implications for climate change. We’ll focus on one of the consequences of warming: melting of the Greenland and Antarctic ice sheets. How might this melting lead to future sea level rise? Scientists are studying geological records of past warming, and associated sea level rise, to see what the future may bring.

An important counterpart to observational evidence is computer modeling, an essential tool for investigating how the climate system works and how it will respond to continued greenhouse gas buildup in the atmosphere. How do we know that a model is accurate? One way is to consider a past event, enter the historic climate data, and see if the model successfully “hindcasts” the event in reasonable detail. Ever more detailed, today’s supercomputer models can even help identify the potential causes of climate events on a regional scale, as climatologist Dr. Michela Biasutti explains using her research on droughts in sub-Saharan Africa.

Climate change is often framed as a future phenomenon, but it’s clear that people are already experiencing the consequences. What are the effects? It depends to some extent on where, and how, you live. For example, in the Pacific Islands, where sea level rise is threatening entire ways of life, communities have come together to prepare. What happens when there’s no place to go? Coastal communities aren’t the only ones at risk; food insecurity may one day threaten us all. And of course, humans are not the only species affected, and some will be at even greater risk in the future.

What happens next? We don’t know, but the answer depends far less on scientific data than on human action—or inaction. The major uncertainty is the future rate of greenhouse gas emissions, which is impossible to predict because it depends on socioeconomic, technological, and political developments. Furthermore, the risks posed by different kinds of natural phenomena vary widely, and similar events can have very different consequences depending on where and even when they occur. How will the effects be distributed, and how resilient are we? Hurricane Sandy provided lessons about our willingness to accept and plan for a future where severe climate events happen more frequently.