Ed Ruscha Paintings, Gagosian Gallery, London

While at art school, in 1957, Ed Ruscha chanced upon the then unknown Jasper Johns' Target with Four Faces in Print magazine; it was to be instrumental in his decision to change from a training in graphic art to painting. "Art," he said, "has to be something that makes you scratch your head."

Born in Omaha, Nebraska, in 1937, Ruscha moved first to Oklahoma then to Los Angeles, where he studied at the Chouinard Art Institute from the mid-1950s to 1960. By the early 1960s, he was already known for his paintings, collages, and printmaking, and for his association with the Ferus Gallery group, which included artists such as its co-founder Edward Kienholz. The irreverence of Pop Art was to be a major influence on his paintings, into which he had begun to incorporate words and phrases.

In the 1980s, these were often set against a mysterious light cast by an unseen source. Like Warhol, Ruscha was born and raised a Catholic, and the tropes of religious art were easily appropriated into his work. Incandescent light, which, in the painterly tradition, represented the divine presence, was employed to highlight not the sacred, but emblems of American modernity, from Hollywood logos to gas stations and the ubiquitous signage that became the basis of so much of Ruscha's work.

Semantic games also became his forte, and his wry choice of words, often in black and white, and non sequitur phrases that seemed to owe as much to the sign painter as to the history of art, created a perpetual interplay between language and the image.

For this show, he has revisited sites, buildings and views of Los Angeles that formed the basis of Course of Empire, made for the 51st Venice Biennale – a powerful body of work that seemed an implicit critique on American expansionism, and in which he aired his "doubts about progress in the world, and hopes for the world... my feelings about how things change – and that they don't always change for the better".

He has paired paintings with another version of the same subject in order to create exercises in perception and memory. The sublime is counterpoised with its tarnished opposite in two paintings of mountains, where the large white letters "CO" have been superimposed on to the landscape, and an obvious architectural ledge or wall cuts off the perfect view.

Elsewhere, The Nineties/ The 2000s suggests an increasingly dystopian vision of modernity rendered in a cinematic sequence of dates that, in the first panel, are set against the flush of a sunrise, but in the second are placed against a grey ground made up of tatty bits of canvas, where a timeline simply breaks off into a blank future.

Most difficult to read, perhaps, is the diptych Azteca/ Azteca in Decline (2007). Based on a motif sprayed on a concrete wall glimpsed through a car window when driving to Teotihuacan, outside Mexico City, Azteca consists of three great triangular plumes in red, green and blue, which rise, fan-like, from a central point. Part-heraldic emblem, part- arcane graffiti, they have, by the second panel, Azteca in Decline, collapsed into tattered, post-apocalyptic "crumbs of colour". Crumpled, convulsed, exhausted, they provide a bleak vision, though of what exactly it is hard to say; narratives about historical change, perhaps, or the hubris and decay of empires?

The same questions seem to be raised by Plank 1979, in which a piece of wood hovers in front of a sunset, and its newer companion, Plank in Decline, which, painted in greys and browns, reveals the wood's obvious slow degradation.

That these works are metaphors for the passing of time, for the evolution and dissolution of societies and civilisations, is surely the case, but there remains something cool and dispassionate about them, like an event being observed through a glass darkly.

To 20 March (020-7841 9960)