Standing on the beach at Grand Isle, Louisiana, a barrier island, piece of the outermost limit of the Mississippi Delta, I am struck by the magnitude of the challenge we face trying to grasp what is happening with the climate, or even to convey some small piece of it in words. I overlook a beautiful wide beach (replenished) with fine white sand, and off-shore in the distance you can see dotting the horizon the rust-red outlines of a few oil platforms, pumping crude for delivery to the refineries that lie to the north of where I stand. Closer in, a shrimp boat plies the waters just beyond the line of surf, nets splayed out in the afternoon sun. Everything is peaceful today, yet everything is in motion.
Where I stand at Grand Isle, bedrock lies far beneath my feet. Over millions of years, the Mississippi Delta added more and more sediment as the interior of the continent eroded, layer after layer, flood after flood, and over the aeons this process formed a compacting layer of silt so thick that bedrock here is only a theoretical concept. The porosity of these surface layers, combined with rising sea levels and the resulting erosion, means that the Mississippi Delta region is losing roughly one square mile per week of dry land. An inland migration of people northward is in motion, a retreat from the advancing sea, generation by generation, now underway for decades, and it will likely continue for the rest of this century. For people affected in this way, from the coastal US to Bangladesh, or the Marshall Islands, global sea level rise is personal.
Weather is something we can experience directly through our senses. It is often immediate and undeniable. We can feel rain or snow on our skin, or taste it on our tongue. We can see the effects of the wind as it tosses the branches of a tree next to us. Climate however is an abstraction. It is the average temperature, or mean high tide, or average rainfall. It is the shifting baseline behind those daily weather events. It is in this sense that we can experience weather, but we can only observe climate. Weather is what we see when we look out the window. Climate is the notional baseline about which things fluctuate.
Climate concerns longer spans of time and greater spatial distances than we can see with our own eyes. We can only detect the climate signal through collective rule-based observation, augmenting our senses with technology. We require testimony from others to bring climate into view. Yet this is also why humanity’s knowledge of climate is objective while our personal experience of weather is not. This distinction can be puzzling until we dig into the question of how scientists come to know things about the world, and the nature of objective knowledge. 
The key difference between an experience and an observation is that the former is forever singular and irreducible — and valued as such — while the latter uses systematic methods that will allow it, at least in principle, to be repeated by another, to become in a sense a commodity, transportable and detached from personalities. An experience is, in a very important sense, always true precisely because it is experienced. But, the experience only becomes an observation when it can be shared and re-experienced by another.
A similar point is made by Amitav Ghosh in his 2016 book The Great Derangement: Climate Change and the Unthinkable. Ghosh argues that it is no coincidence that the novel appeared as literary form at about the same time as the emergence of science, because it is truly a literature of observation. Both the novelist and the scientist require the understanding of the essential character of observation as rule-based watching, accompanied by a shared language used to describe what was seen.
Objective knowledge is therefore not truly achievable in isolation. Instead, objective knowledge must be constructed. It lies in the spaces among us, built upon and only understood through an agreed-upon set of rules for observing, coupled with a shared language so those experiences can be compared across time and space.
Consider the case of sea level rise. The US National Oceanic and Atmospheric Administration (NOAA) says that of all the sites along the US East Coast and the Gulf Coast, the tidal gauges at Grand Isle, Louisiana, register the most rapid change in relative sea level. These gauges are attached to docks or piers, and the sensor readouts are collected every few minutes and then made available online. The network of tide gauges form a ring around the coastal US, and many others are deployed worldwide, forming an international pool of knowledge, from which an attempt is then made by scientists to create an objective understanding of the situation.
Along with satellite observations, the tidal gauges allow NOAA scientists to estimate that sea levels globally are now rising on average about three millimeters a year, about the width of one of the folds in the skin on your knuckles. This change is due to a combination of the warming of ocean waters, which causes them to expand, and the addition of outflow from melting ice that resides on land, in glaciers and ice sheets. (If the ice is already floating in the water, then melting it doesn’t change the global sea level.)
Superposed on that average rise in global average sea level, local relative sea level can change due to the land subsiding, or changes in coastal ocean currents, since the average sea surface height is depressed in the presence of those flows. At Grand Isle the land is sinking twice as fast as the seas are rising, giving a total change in relative sea level of about nine millimeters a year. That’s almost the length of the last bone in your index finger. Every year, a steady accumulation of encroaching salt water and elevated risk during storms.
Trying to glimpse the future of sea level rise in Grand Isle means we need to follow the state of the grounding lines of glaciers on the far side of the planet, in Greenland and Antarctica. These are essentially a naturally-formed dike holding back a river of ice that would otherwise flow much faster into the sea. If those grounding lines give way, the recent rate of increase in sea levels would accelerate dramatically. The fate of our coastal cities is therefore determined partly by what goes on in the dark sea bottom a continent away, in frigid waters a kilometer or more beneath the surface, as rock, gravel, sea, and ice negotiate with one another.
Around the globe, underwater autonomous robots sample waters, acoustic sensor arrays measure the ocean’s temperature profile, and humans dive to see with their own eyes the state of things. Elsewhere, they walk on Antarctic ice sheets and Arctic tundra, or fly in carefully mapped transects across the Greenland icepack to measure its shape and thickness, to talk to one another and our machines, to mine our growing midden pile of data. This is a large international conversation, the largest and most complex collaboration in human history, aimed at trying to puzzle out the objective state of the earth’s climate, to fit all that data to idealized models and to thereby turn those terabyte arrays of numbers into human knowledge, an image of reality in the collective mind’s eye of humanity.
Therefore, our fate depends upon events that are happening, or will happen, at places that only a very few of us will ever see with our eyes, or stand upon with our feet. Many of these places will in fact be monitored by our machines who will gather the data for humans to analyze. We have to trust others to act as sentinels in those far off places, to keep an eye on them for us, to use those machine extensions of our senses to help us see in those dark places, and to make visible the invisible, because if those frozen stores of water give way the world will quickly turn wild indeed.
Anxiety about the future, and an abiding sense of alienation and loneliness, are aspects of the modern human experience. Like pre-literate humans awakening to their own mortality, our growing social awareness is awakening to the mortality of our civilization. The good news is that humans are an adaptable species, and inventive. Yet this genius for invention is precisely what brought us to the current situation. We have now created the first truly global civilization, but based it upon an unsustainable foundation: fossil fuels.
Historians writing in some distant future (hopefully) will see fossil fuels as an easy path taken early on in our technological development, while a better path was followed once the dead-end on the fossil fuel track came in view. There is reason for cautious and guarded optimism about our prospects. Surveys suggest that most people understand what’s at stake, and want our leaders to do the right thing. But it will not be easy, and it will not happen quickly. It will be the work of generations.
If it’s true that, ultimately, a culture lives in the brains of those adults who choose to take part, then the same is true of our shared sense of objective reality. The modern scientific idea that individuals cannot immediately apprehend reality, but instead we must strive for objectivity through the creation of systems of shared subjectivity, all this points to a central vulnerability of human society. Without a shared reality that allows us to map individual thoughts and experiences, one onto the other, we risk more than just a breakdown of our current fractious political system, but the bonds that allow us to build civilizations in the first place.
For all its faults, and they are legion, modern civilization allows most of us to spend our days doing more than growing or hunting the food we eat, fighting off predators and other people trying to steal that food, or scrounging for our night’s shelter. This achievement also means that we have to trust strangers to do the right thing most of the time, to tell the truth as they best understand it, most of the time. To act in good faith, most of the time. Every system has its cheaters, but a system that doesn’t believe in the possibility of a shared reality can’t last very long because the non-human physical world — ever present, inescapable, and taking little cognizance of human desires or wellbeing — will eventually overwhelm us.
Standing on the dune break at Grand Isle, next to one of the numerous signs that declare ‘No motorized vehicles!’ I step aside for an elderly gentleman in his motorized golf cart. He sputtered forward up the hill, lugging his beach umbrellas, canvas chair, and drinks cooler, out onto the wide strand proper, a determined human being, head down, either not noticing what the signs say, or choosing to ignore them. And then I notice a half-dozen golf carts like it are already parked along the beach, scofflaws every one. The scene is dotted with colorful umbrellas and a smattering of sun seekers. I wave at them, and they wave back. It’s early June, before schools have let out and the main summer season hasn’t yet begun, so the beach is not crowded at all. In fact it’s nearly empty. A quiet calm day.
Acknowledgements: My trip to Grand Isle, Louisiana was supported by the Center for the Liberal Arts at William & Mary.
 See Lorraine Daston and Peter Galison Objectivity (Zone Books, 2010).
Image at top: Satellite imagery of Grand Isle, Louisiana. Source: NOAA.