Climate change directly affects the geographic range of species, the timing of species life cycle (phenology), the population dynamics of species, the decline and extinction of some species and the invasion of other species. Plants with C3 photosynthetic pathways are expected to benefit more than C4 from CO2 enrichment. However, rising global temperature may give competitive advantage to C4 plants than C3. This differential response of C3 and C4 plants will alter crop weed interaction because of the fact that majority of weeds are C4 and most of the food grain crops are C3. Higher levels of carbon dioxide could stimulate the growth of some weed species and greater production of rhizomes and tubers in perennial weeds making them difficult to control. Warmer temperatures will accelerate the rate at which day degrees accumulate, so the life cycles of some plant species may accelerate. As a result weeds are likely to mature and start to decay earlier.